I've been a GM through every edition since AD&D. That said, I can honestly say that I've seen everything from awful to amazing in third party content. I find this suppliment to Waterdeep: Dragon Heist's First Chapter "A Friend in Need" is pretty wonderful in fact.
1). First off, the PDF comes in both a nice fancy version and a plain text version. Personally, I printed the plain text document, which I find less distracting and can easily make notes on when running a game. I highly appreciate the graphic design and layout of the "fancy" version, however.
2). Extra Content. The first part of the suppliment is dedicated to enhancing content and maybe even covering some gaps in the adventure. You'll have to buy it to get into the details, but I will say the Sava game was especially helpful and helped make a memorible NPC the group has come across a couple of times now.
3). THE DM'S NOTES - This was gold for me. If you read the "A Friend in Need" DM's Notes over, print it out and have it ready, you ARE a well prepared DM for your game. It covers, step by step, what happens in bullet point format that is easy to cross off, highlight, and make notes.
4). The Monster Stats. The suppliment gives you all the relevant stats for your encounters in the chapter. This seems like a small thing until you start to wrestle with several books when your DM'ing the chapter. Once again , I simply print off the suppliment, read the monsters/villiams over and make any notes I need.
5). A Map. You get a nice map of the Yawning Portal to boot.
I would highly recommend buying the entire DM's resource set for the Waterdeep: Dragon Heist book.
[5 of 5 Stars!]