Disclaimer: I received a copy of this book for free because I helped proofread a portion of it. I did not see the majority of the book until it was released, however, and was very excited to see it in its entirety.
I'll work through the chapters one-by-one, then give an overall assessment at the end. I'll also provide a TL;DR at the very very end. This is a long one, as befits a long book.
1. RACES: The races are all narratively interesting, and I feel they help fill different niches than much of the official racial material. They each provide plenty of description of each race's society, typical mannerisms and personalities, etc. to help players get a feel for what that character might be like. The races' abilities also seem to mesh with their themes, allowing them to fit into some obvious archetypes for classes but not excessively restricting them or making a class feature redundant with a racial feature. My personal favorite is he shadar-kai, which absolutely screams out for me to make a rogue, though its abilities make it excellent for any stealthy character. I like this version of the shadar-kai much better than the one WotC put out in its Unearthed Arcana article.
As a general note about the book that I first noticed in this chapter, several individual items in this book have been previously published in portions or in their entirety by contributors to this book, such as the dhampir, hengeyokai, and saurian races (all three by Benjamin Huffman, AKA Sterling Vermin), so if you're interested in only one or two items from the table of contents, try searching for those individual items elsewhere. I'm not sure exactly how many items from this book can be found elsewhere (since I don't make a habit of buying, reading, and remembering everything out there on the DMs Guild and elsewhere), so I can't provide a comprehensive list. The designers listed on the credits page are Benjamin Huffman, Chris Bissette, Jeremy Forbing, M.T. Black, Patrick E. Pullen, and Scott Bean, so start your search with their work.
2. CLASSES: Each class from the PHB gets 2 new archetypes here. The authors worked to try to include archetypal options that add more complexity, but to match those up against straightforward archetypes. They don't do this for every class (some classes have 2 archetypes of low to moderate complexity), but several of them definitely do. Fighter, monk, rogue, and warlock all have an option that definitely adds a lot of complexity to the class, paired up against a simple option.
Many of the classes also get at least one d10 table of trinkets. It seems that each trinket table is intended to go along with a particular archetype, though I can only base that assumption on formatting and visual organization of the book rather than anything explicitly stated.
The ranger archetypes are listed as "conclaves," which is the word used for the Unearthed Arcana revised ranger archetypes, while the PHB ranger's archetypes are called... well... "archetypes." This creates a little confusion about whether these archetypes are intended to blend with the PHB or UA ranger, though I know there have been guidelines on how to make an archetype for one fit the other. I'm probably overthinking this.
I haven't done an in-depth evaluation of the archetypes here to see how they fare as far as balance goes - my assessment is more based on a general feel of balance (which WotC developers have also said is something they consider when "balancing" an archetype). That said, all the archetypes seem pretty well balanced to me, though of course some will shine brighter and seem more powerful in certain types of campaigns or under certain DMs. The archetypes are also all interesting narratively - they have a place in the world, rather than being an excuse to smash interesting features together or requiring a character’s story to come almost entirely from their race/background. I like them all, and I can already envision a few characters using them.
Here's a little idea of what some of the first few archetypes have to offer, with my thoughts on fun race/class pairings. This will be the longest part of the review, so if you just wanted an overview, go ahead and skip to my review of the next chapter.
-Barbarian: The ravager barbarian sticks close to the idea of a raider, focusing on mobility and power, while wildrunner brings in Feywild elements that would make an elf (or hengeyokai) barbarian an interesting choice. I'd like to make a ravager soon... maybe as a saurian.
-Bard: The disquiet bard gives you the option to be edgy without going to the crazytown that Xanathar's college of whispers bards go to. A dhampir would be a great fit for this. The college of grandeur gives you some powers reminiscent of 4e's warlord class, allowing you to grant allies temporary HP and to empower them to act out of turn. Warlord aficionados will likely gripe that it's not enough, but the only way to satisfy them would likely be a full warlord-style class, which has been done elsewhere (sometimes quite well!) in 3PP products. This is a great way to give the bard class a smattering of warlord-esque abilities, and might be the first bard archetype I've ever wanted to play! Maybe I'll make a nixie grandeur bard.
-Cleric: The description of the exorcist cleric domain screams Ghostbusters meets Men In Black. The archetype's features don't quite make it there, but perhaps that's a blessing given the different genres. It does clock in a solid, powerful archetype against unnatural entities that I'd be happy to play (and might even name my character Jay Venkman for kicks and giggles... of course, that name doesn't seem appropriate for the myconid race that makes for a great pairing here, or for the dhampir race that makes an interesting thematic choice but doesn't have the ability scores for it). The prophecy domain, on the other hand, does what you'd expect regarding support and foretelling (thematically but not mechanically similar to the divination wizard), but also has a physical ailment that, while burdensome, also grants you some benefit as well. This benefit usually makes sense with the ailment, but not always... I guess that's where "it's divine magic" becomes the answer.
There are just so many archetypes, I can't summon the energy to process and review them all, but I do want to hit on the four that I believe have the most complexity since some people might be looking for some meatier options.
Rogue gets the Daggerspell Guardian archetype. It's basically another version of arcane trickster, but where the arcane trickster dabbles in wizardry, the daggerspell guardian can choose to dabble in sorcery or druidism. You get sorcery points that let you do rogue-ish things, and eventually you pick up either metamagic or wild shape. Lots of things to keep track of here: spells, sorcery points and the things you can do with those, metamagic abilities or wild shaping (you can pick from a lot of different animals with this, which means you want to be familiar with at least a handful of go-to beasts if not more), plus remembering all the normal things a rogue wants to keep track of like rules for sneak attacks, stealth, etc.
Warlock gets the Elemental Potentate patron. If you were to ask yourself the question "Hexblade patron warlocks really expand on the pact of the blade, but what patron would really expand on the pact of the chain?" then you would probably get this archetype. Think sha'ir. You get a special familiar that can do a few different things for you (potentially more than a typical familiar), but its primary task is to fetch you different spells over a long rest, from either the warlock or sorcerer spell list. This means you have to be familiar with all those spells that you could choose, and try to plan ahead for what you'll need that day. You also eventually get the ability to summon minor elementals, which means you get to control (well... control-ish) minions. Add in more eldritch invocation options and you've got a whole lot of options and things to keep track of in and out of combat. [Side note: I like that the list of benefits granted to the different pacts acknowledges the design space of other pacts beyond the core three and grants a benefit for those non-standard pacts.]
Monks... I love monks. They're pretty straightforward (except for that Four Winds tradition that catches so much grief, right?). The Way of the Thousand Steps is set up like Four Winds, with various disciplines you can use by spending ki points. Its disciplines are geared around movement, and it has a lot more disciplines to choose from than Four Winds. (Four Winds has 17 disciplines. Thousand Steps has a whopping 51.) Four Winds only ever lets you know 5 disciplines, while Thousand Steps will let you know up to 9, and while Four Winds says "these disciplines are your only archetype features," Thousand Steps still gives you other features that all deal with movement. These other archetype features are probably to make up for Thousand Steps seeming to follow Four Winds' convention of having disciplines cost ki equal to the spell level+1, which seems to be a major point of contention. So... yeah, the usual monk stuff, plus a bunch of disciplines to choose from and use, plus other archetype features. This archetype almost takes the cake in this book for complexity, but I'd say it's a close second or maybe a tie with, ironically...
Fighter. The bastion of banality, finally getting something with even more options and complexity than the Battlemaster or Eldritch Knight archetypes in the form of the Talaric Battlemind. Yes, "battlemind" as in psionics - thankfully it doesn't become quite as elaborate as either version of the Mystic class WotC presented in Unearthed Arcana. It actually feels a bit more like the Four Winds (and Thousand Steps) monk's disciplines. Now you get to keep up with cantrips, psionic manifestations (and how many of them you know), a psi limit, and psi points. And your casting stat is Constitution, even though you don't really cast leveled spells - there are 38 different manifestations to choose from, and only one of them that I see ever says "you cast the _ spell." Oh, and every time you spend psi points on a manifestation, you (the player) get to remember to regain HP (on your character). If you want a fighter with complexity, you got it right here.
3. BACKGROUNDS: These backgrounds provide solid story hooks for characters, and several of them encourage players to collaborate with their DM (which I think is always a great thing to do when creating a character, and any feature that encourages/requires that is a good one in my book). These might not all be good fits for every campaign, but they fill in gaps among common backstory tropes that official WotC publications don't fill.
I feel like the Feral background provides a slight mechanical penalty (in the form of reduced languages) that isn't offset by any other feature in the background, but it does offer one of the more classic barbarian backstories, and they're not exactly expected to be the linguist of the group, so I guess it's ok. The player chooses the background willingly, knowing the penalty, so presumably it's worth it to them.
The mortician background gets a little dark, and recommends the player get DM approval. I'm not a huge fan of opening up these sorts of depraved/evil options to players (even with that caveat) in a standard book - honestly, I'm not a huge fan of some official WotC class/archetype options for the same reason. Perhaps it would be acceptable in a book geared more toward adults and depraved/evil themes, where everything needs that caveat... or perhaps I'm just a prude.
4. EQUIPMENT: Solid stuff all around. No standouts as exceptionally good, bad, or broken here, but also no waste. Many supplements I see just write up the same stats and slap new names on the armor or weapon. Not here – you get your money’s worth.
Nothing much to see in the armor department. 5 new pieces of armor: 1 light, 2 medium, and 2 heavy. One is cheaper than other options in its class, one is better AC than other options in its class, and 3 offer minor mechanical benefits over their PHB counterparts for increased cost. Three of them have some roleplaying/narrative value.
The weapons get a little more interesting. All but one of the new weapons are martial. Many of the weapons are classics making a comeback, and most of them bring a small new mechanic to the table. There are of course several popular double-ended weapons that make a comeback, as well as composite bows and repeating crossbows. In a book that has the shadar-kai as a playable race, you also expect to have the spiked chain, and this book doesn’t disappoint. All of these are great additions to the game that won’t drive anyone up the wall trying to figure out how each one works.
The adventuring gear is sensible. Some new tools and instruments, some new spellcasting foci, some books that make use of downtime rules, etc. The sunstick returns, but at a much higher price than I remember it costing… no worries, torches still work until I’m higher level and have boatloads of gold lying around. We also get potions of magical restoration (aka blue potions or mana potions) and tanglefoot bags! I think my favorite entry is the dictionary – mechanically it has a downtime effect, but I just imagine an adventurer carrying around this big tome of words to correct people on their spelling, pronunciation, and word usage – it’s essential equipment for every wizard, especially the academic lore wizard presented in this book. (I kid, I kid!)
Trinkets are a thing. I don’t typically use them in my games, but the few times I have, they have encouraged wrinkles or additional layers to character backstory, and I’m not opposed to that. 100 more trinkets = 100+ new character story ideas. I’d say that’s a fair use of 3 and a quarter pages.
5. FEATS: Lots of good options here! I don’t feel like any of these are “fluff” feats that only add roleplaying value or are too situational to see much use. There are 21 total feats, 11 of which are “half feats” that grant +1 to an ability along with other benefits. The ability bonus granted by these half feats is always specified, though one of them gives you the option to pick between two abilities to increase. The feats predominantly cater to combat, though several would be useful for social interactions, and a number of individual features within the feats could be useful as part of exploration. All of them are accompanied by a short blurb that uses non-mechanical terms to describe the abilities granted by the feat in in-game terms, which I think is a nice addition, as it reminds the player that these feats are a part of who their character is as much as what they can do.
6. SPELLS: I don't often play spellcasters, so I can't even begin to process all these spells - I need some digital spell cards, or a way to sort through the spells by what I'm looking for like D&D Beyond allows. (If the creators wanted to... say... upload these spells to the Homebrew database on D&D Beyond, I would grab them in a heartbeat!)
There are 134 spells spread over 34 pages, including some evocative art. The ones I proofread all seemed interesting and useful to various character types, though Sorcerer and Wizard get a lot of them on their lists, even compared to other full casters. Since there's no way I could adequately review all of those spells without writing more pages about them than they take up in the book, here's some info you might be interested in - namely, the spell school/spell level breakdown.
Don't like lots of numbers? Here's the number-free version: The spells favor transmutation and evocation while seeming to overlook divination, and they slightly lean toward low-level spells over high-level ones (which makes sense).
-Abjuration: 15 spells. (2 cantrips, 1 1st, 3 2nd, 3 3rd, 3 5th, 1 6th, 2 7th)
-Conjuration: 16 spells. (1 cantrip, 5 3rd, 1 4th, 2 5th, 3 6th, 2 7th, 2 8th)
-Divination: 9 spells. (2 cantrips, 4 1st, 1 6th, 1 7th, 1 8th)
-Enchantment: 14 spells. (4 cantrips, 2 1st, 1 2nd, 2 3rd, 1 4th, 1 5th, 1 7th, 1 8th, 1 9th)
-Evocation: 23 spells. (1 cantrip, 1 1st, 3 2nd, 3 3rd, 4 4th, 5 5th, 1 6th, 4 8th, 1 9th)
-Illusion: 13 spells. (3 cantrips, 2 1st, 2 2nd, 1 3rd, 2 4th, 1 5th, 1 6th, 4 8th, 1 9th)
-Necromancy: 17 spells. (2 cantrips, 2 1st, 2 2nd, 2 3rd, 1 4th, 1 5th, 4 7th, 2 8th, 1 9th)
-Transmutation: 27 spells. (3 cantrips, 8 1st, 5 2nd, 2 3rd, 1 5th, 2 6th, 1 7th, 2 8th, 3 9th)
That means there are 18 cantrips, 20 1st level spells, 16 2nd levels, 18 3rd levels, 9 4th levels, 14 5th levels, 10 6th levels, 11 7th levels, 12 8th levels, and 6 9th level spells.
Bards get 48 new spells on their list, clerics get 28, druids get 33, paladins get 14, rangers get 18, sorcerers get 63, warlocks get 38, and wizards get 65.
7. BETTER GAMING: This chapter is relatively short and contains no game mechanics. It is solid, helpful advice for people new to the game (and some people who aren't new to the game... often called "problem players"). However, I question how many people new to the game will have their hands on this book. As for the advice that would be most applicable to people who aren't new to the game... well, if they haven't learned those lessons about what makes them a problem player by now, I don't know if this or anything short of direct confrontation/communication by members of their gaming group will help. On the whole, I feel this section could have been left out and the book would not have been worse for its absence.
8. CHARACTER NAMES: The preamble to the book says this chapter was a late inclusion, and it shows.
Many people were furious when it was announced that Xanathar's Guide to Everything would "waste pages" with a 17-page-long section on names, but those 17 pages were also interspersed with 9 pieces of art (ranging from small to one-third-page in size, with many being one-quarter-page) indicative of the culture each was adjacent to, and the names were helpfully presented by (fantasy) race or (real-world) culture, as well as by gender. The names were listed alphabetically in four columns with only one name per line per column, and presented as random tables to roll on or choose from, with 50 names per table. Regardless of your opinion of the need for the names, they were well organized and presented.
On the other hand, the 7 pages of names in this Player's Companion have 4 one-quarter-(or less)-page pieces of art interspersed, though it's not clear how the art relates to the names. The names are also not broken down by anything other than forenames and surnames - not by race, culture, or gender... they are at least presented alphabetically, for whatever that's worth. They are presented in two columns on each page, with 5 to 7 names on each line of each column – however much would fit between the margins. Reading them makes me go cross-eyed. On the bright side, since this is a PDF that didn't have a predetermined page count (that I know of) like WotC's published books, these aren't "wasted pages." If you want to use them, great! I'll save my eyeballs by sticking to the earlier chapters.
OVERALL/FINAL ASSESSMENT: The book's appearance is professional, and they used great art pieces to help bring it to life. There are a few layout decisions that are a little bit questionable (mostly regarding tables and spells that spill over onto other pages), but not enough to be a real problem outside of the chapter of names. I'm also a bookmark-man... I like my PDFs thoroughly bookmarked. At the time of writing, this PDF only has bookmarks for the start of each chapter. Easy enough for me to fix on my own, but in this day and age I don't feel like I should have to. The book is 170 pages long, plus a page each for the title, credits, table of contents, and introduction. Chapters 7-8 combined (the two chapters I could do without) are only 15 pages long, so you're really getting 155 pages of awesome stuff... at its price of $14.95, that's over 10 pages per dollar (over 11 pages/dollar if you count the last two chapters back in), and it's well worth it! I'm looking forward to playing my shadar-kai acrobat rogue... or will I make a saurial ravager barbarian first? Or a nixie grandeur bard, or a myconid exorcist cleric, or... or... Too many good options in this great book!
TL;DR: The last 2 chapters are the worst part of the book, and their only problems are unimportance and poor organization/format, respectively. The other 155 pages are great! 5 stars! Worth the cost!