It began with the "Monster Manual." You expect there to be a book full of monster stat blocks. PFRPG has that, and this is it. What you get in addition are superbly drawn pictures of each monster, well designed (but not distracting) layout, plenty of use of the creature advancement and templating system, and some excellent use of pre-existing d20 OGL resources outside of the basic SRD monsters.
The only thing that I think is missing is a simple trade-off. I want more fluff, but this book is so full of creatures that there isn't enough room for it. Instead, Paizo is producing a line of fluff-heavy creature books such as Classic Horrors, Misfit Monsters, etc. which I'm eagerly gobbling up in order to add spice to my campaign world.
The stated goal was to redeem some of the creatures from the classic game that just never quite worked. On almost all counts, this book succeeds. The Flail Snail is, for the first time, an interesting creature worthy of use. That's saying quite a lot given that it was the go-to example of silly monsters. The Flumph is a creature that Paizo has a history with and they continue to improve on, here.
I found myself unimpressed by the Adherer (which had a lot going on, but is still a sticky mummy) and a few others, but let's face it: I've never had a bestiary of any sort that I felt I wanted to make use of in its entirety, and the story ideas, examples and backgrounds in this book will keep me quite happy for some time to come.
A lot of this review is going to be negative, so let me start out by saying, I love this world, and this is an excellent source book. I'm nit-picking because I feel there's so much to improve, but where they started is amazing.
Also worth note before I get started: there is a new version coming out sometime in early 2011, so if you can get this book at cover price, it's probably still worth it, but some of the crazy prices I've seen at 2-5 times the cover price are definitely not worth paying. The new version will be an update to the latest Pathfinder rules (this edition is just OGL-compatible), which means you can expect the sections about character classes to change, but that's probably about it. There's not much "crunch" in this book, as one might expect, and so a rules update is kind of tangential to its content. There is also a book about the Inner Sea region coming out (Pathfinder Campaign Setting World Guide: The Inner Sea (Revised Edition)), so if you are specifically interested in that region, you might want to go that route either alone or in addition.
Now to the review. The cover art is blurry. An odd defect, given Paizo's usual standards, but I'll assume this is an artifact of their early rush to print when they were just getting started in the post-Dragon/Dungeon magazines era.
Inside, the book is beautiful. The art is spot-on and really gives a sense of the world. The poster map that comes with the book is excellent, if somewhat more sparse on detail than I would have expected. The races are wonderful. I especially love what they've done with gnomes: a race I previously couldn't care less about is now one of my favorites!
However, as you start to read, you definitely get the sense that someone forgot to write the introduction chapter. You end up having to essentially read the entire book cover-to-cover in order to make sense of the world. An intro that told us, "This is Golarion. Most of the action will take place in or within 1000 miles of the Inner Sea, a temperate sea that acts as the divide between the two major continents...." and so on. Not an in-depth view of the countries and peoples. There's plenty of that later on, but just a quick intro so that I don't feel lost the first time a major country or god's name is dropped as if I'm supposed to know what it means. I actually bought this book because the Adventure Paths were giving me the same lost feeling, and it solved that problem, but only after I slogged through most of the book (starting with the History chapter helped a bit).
In the writeups of the races, I have roughly the same problem. You almost have to start reading them from the middle, because that's usually where they get around to explaining who and what they are, rather than how they found themselves in some historical calamity, but again these write-ups are brilliant. Gnomes are much improved over their D&D roots, but every race has some excellent flavor to it. Elves will definitely be interesting. They're not really very nice, and I like that about them. In fact, between the Elves and the Drow, it's actually hard to choose a group to support, but the Drow have a point in feeling abandoned and wronged by the other elves.
Humans are my favorite race, however. I love the fact that they're not just the everyman white, European analog with a few ethnic minorities thrown in for flavor that most fantasy games have. They're a diverse mix of humanity both fantastic and realistic, and the Inner Sea is ideally located to be a mixing pot of Arabesque and pseudo-Eruopean influences of many flavors.
Overall, this is an excellent resource. If you're looking for a fully realized world on which to base a campaign, this is definitely it. If you're looking for a quick reference, then I suggest picking up the smaller Chronicles books such as Gods & Magic and Guide to Absalom that suit your needs.