8/22/18 Update: The book is objectively better now that the old, painted figurine art has been replaced.
Disclaimer: I backed the Kickstarter that created this product and paid the full price there for both the physical and digital copies of this product. As a crowdfunding backer, I received a physical copy well in advance of its wider public release (and, in fact, had it at the time of this review). I was given a free copy of the digitial version of this product here on DriveThruRPG for the purpose of this review.
Now, there's a lot to get into with this product, and I'm not sure I actually have the space here. That's why my full review of this product can be found over in the Discussion tab, where I go into great detail about what you can expect to find throughout this particular book. If you're not sure whether or not you want this product, I strongly recommend hopping to the other tab and reading that in full - by the end, you should have all the information you need.
As a briefer summary, however, this is primarily a GM product. While it includes some player-focused options, including archetypes, feats, and the like, many of these are most appropriate for things the players will face (unless your game is all about being servants of Mythos things, in which case this is the best Pathfinder supplement on the topic, period).
Aside from common things like archetypes and spells, this book features grimoires, unique artifacts, a new Dread mechanic, and best of all, Elder Influences. These are a new type of challenge within the game, about halfway between a monster and an environmental effect, and they represent the way that Mythos creatures can impact the world by their mere presence. Reality itself gets weird when these things show up, and many of them are a serious challenge for even high-level parties. For example, Hastur the Unspeakable has an effect where each of his manifestations can only be observed - and affected - by one person. Sure, the raging super barbarian may be able to smash his way through, but will the squishy sorcerer find it quite so easy? Perhaps not. And he's far from the worst threat the Outer Gods and Great Old Ones offer.
This book is worth full price for the Elder Influences alone, but they only comprise a part of this hefty tome. There are also a variety of new creatures, new races (including a cat race, which works better than you might expct), and references to mythos stuff published in other books so you don't have to go digging through all of them.
Put simply, this book is the best Mythos-themed product for Pathfinder. Period. If you want Cthulhu, or anything related to Cthulhu, this is the book to get. For manufacturing reasons, physical copies of this weren't widely available at the time of this review. I have mine, and they should be more widely available to the public once the second printing arrives, but when I wrote this you may be limited to the PDF version. (That said, the hard copy is pretty nice - good quality paper, a cloth bookmark, and crisp printing for the illustrations.)
Overall, I'm extremely happy with this product, and I think it was absolutely worth my initial investment. The rules content (i.e. the huge majority of the book) is fabulous, and the updated art is much nicer than the old miniature pictures were. I rate this product 5/5 and strongly recommend it to everyone who wants more Mythos in their Pathfinder.