After spending about an hour reading through pages, looking at the artwork and taking a few moments with NPC stat blocks that jumped out I was able to turn some notes into an initial review for those curious…
This is a fantastic resource. I was initially disappointed to hear that content was limited to the Core Rulebook. After reading and reflecting it is clear why this decision was made, and I support it. My primary concern was potential limitations suffered by the NPCs. Limitations that players would likely not suffer. And don't get me wrong, the breadth of material that has arisen since the Core Rulebook's release is not trivial. However, much of it is relatively painless to swap out. A feat here, a spell there, heck, even an archetype or race. You get the point... Limiting the NPC Codex to core content allows it to maximize compatibility. The most glaring loss are the Advanced Player's Guide classes and archetypes. For this reason I will not be upset to see an Advanced NPC Codex or NPC Codex 2 in the coming year(s). I'm not holding my breath though.
Many of these NPCs can be brought into a campaign with no additional prep time. Unfortunately, when an author is given even less than a full page it is impossible to add in something like a preferred Wild Shape stat block for an NPC. For many circumstances the book will be enough. In others, you're going to have to make some additional notes, even if it is a post-it-note. At the very least an NPC will provide a solid jumping off point. There are hundreds of character builds at various levels of progression. If you wanted to tack on a few more levels (don't forget the option of multiclassing) it won't take as long as building a character from scratch.
The art is really great. I mean, it's really great! Just like the bestiaries, its a pleasure simply to look at the familiar races of Golarion. I'm disappointed that not all races received the same treatment. Many, but not all, of the Half-Orc illustrations leave something to be desired. This is something that a half-orc lover is most likely to notice though ;)
The Appendices could be very useful on the fly, particularly the "Encounter Groups" where you pick a theme and encounter CR. Then flip to the appropriate pages and your NPCs are prepped and ready to run. The idea to build the druid with domains and allow you to swap the domain out for an animal companion from the appendices was a great idea.
My concluding "initial impression" is that this resource is like the legos we had as kids. Sometimes you get a kit that's just perfect. Some kits inspire you to create something more personal to you. And still other times you pull ideas from multiple sources and build something completely new. The content in NPC Codex is nothing short of inspiring and time saving. Kudos to everyone involved with this project. Paizo, you and your contributors put a lot of love into your work and it shows.