What do you get?
This collection of 66 NPC stat blocks (although in reality, it's more than 120 since most of these come with background for two npc's) fill the "CR Void" found in the npc stat blocks that already exist. Wizards of the Coast has done a wonderful job providing many a dungeon master with monsters, but when it comes to NPC stat blocks there are many levels entirely lacking an NPC. While that's not exactly a problem, with the NPCs found here to fill the gaps, your job as a DM has become much easier.
What really works in this book?
In terms of presentation (artwork and layout), NPC variety, and the obvious number of passes to ensure this product is worth the asking price. The NPCs as presented all work perfectly well, and the variety of options that come with them will help inspire DMs find interesting ways to fit these characters into their game. For example, Delver has a variant "Deep Miner" which comes with suite of abilities to swap into Delver to provide more variety.
What doesn't work in this book?
Personally I would have made two changes: every NPC comes with two samples. They all have "Sample TYPE NPC" (for example "Sample Diplomat NPC" or "Sample Warchief NPC". Since this is the same for every block, I would have just replaced this with their names. "Dreevil" and "Varfu-Lok of the Iron Fur Tribe" for warchief, as an example. Additionally each NPC has one stat block but a few of these have different races. For example, Alchemist has a generic NPC and a Kobold NPC. The stat block doesn't account for kobold stats (which is fine since the WOTC ones don't) but I would have included the appropriate adjustments at the end of the kobold fluff. Page 282 of the DMs Guide details these stats, and linking just the relevant information here would've helped customize the NPCs more.
I love this book. I want it printed so I could have it at my table. While there are the two changes I would have made to the individual enteries, the book as a whole is entirely cohesive. It makes me wish I could cut the NPC sections from my Monster Manual and Volo's Guide and insert them along side these NPCs at the table.
Edit: I do want to point out there IS a sidebar on altering the stats based on race, I only wanted to point out that I would have moved the relevant lines near the apporpriate blocks, so if you are hoping that table is in there, it is! Also, the author pointed out that 35 was his old one, and I went back and counted. There are more than 60 stat blocks with samples for each. That's around 120 characters!