The title is misleading, as was posited by many during the product preview, and mealy-mouthedly denied by Paizo. This is a Golarion book, period, which has no place in the core line, and the contents consist of an insultingly large percentage of reprints. Shameful, really.
Like others have said, this is the single most useful third party book available to Pathfinder GMs. Absolutely fantastic stuff, well done, well executed, and accompanied with great art. Five most deserved stars.
On the one hand, there are a good couple of gems and some very inspired entries throughout this book and, in truth, two stars is somewhat unfair, especially for the PDF. But, personally, I feel they're warranted, as this book seems to signal a shift in Pathfinder's direction, and one that I'm not happy about in the least.
First off, while Occult Adventures is readily available for consultation online in the SRD, I did not like to see a sizable number of monsters using rules from that book. It's one thing to have spells or feats from, say, Advanced Player's Guide or Mythic Adventures, as those are books that complement the game as a whole and present options for all classes, whereas Occult Adventures is a very specific niche that not all players will want to incorporate in their campaigns.
Secondly, I did not like the muddled product identity Paizo is showcasing here, with monsters such as the Android, Gray (Roswell), Reptoid (Reptilian) and Robot. Even the Annunaki seems more at home as the antagonist of, say, a Legion of Super-Heroes comic book. It's one thing to want to provide the tools for varied and diverse fantasy campaigns - in which constructs, space aberrations and even clockwork creatures can easily work without sticking out like sore thumbs - but quite another one to insert borderline hard-SF or contemporary conspiracy theorist creatures on a lark. Expedition to the Barrier Peaks this is not, and let's endeavor to keep it that way, please.
Thirdly, snake-bodied weasels with boar tusks? Serpentine bulls? Shark-headed sea serpents? Wolf-headed sea serpents? A chinchilla with a bat wing on the tip of its tail? An octopus with three shark heads? Really? Unless you have an absolutely amazing hook or a compelling campaign seed, why bother putting out this silly, uninspired dross? You can do much better than this, Paizo.
It's been a while since it took me so long to digest a Pathfinder book, and boy, did Unchained ever keep me digesting. More optional rules than you can shake a stick at, to be implemented in modular or wholesale fashion, to tweak your game to your heart's content, and with top-notch art throughout, to boot. Excellent work by Paizo and one of their finest offerings in a while.
As for the negatives, the only thing I can really point out is that the writing can be somewhat scattershot and unfocused in a couple of reasonably complex sections, which would have benefited greatly from examples or bolded formulae.
I am extremely disappointed in this product, and glad I only purchased the PDF version. As other reviewers have pointed out, the new classes are poorly balanced when compared to the preexisting ones, and would be a better fit for a book like Unchained, much like Unearthed Arcana's gestalt characters.
There's far too many feats and toys dedicated to these new classes, which drastically lessens this book's universal appeal, and even though some contrivances are provided to make some of said feats accessible to other classes, it ends up painting the book's title as a misnomer - it isn't an "advanced class guide" any more than it is a guide mostly about ten new classes, that are "advanced" only in the sense that they're more powerful than the others.
It's my fault for not properly researching the book before buying it, so buyer beware if you're expecting to find a literal advanced class guide, as opposed to a lengthy introduction to ten new ones.
