I don't usually review products, but it makes me sad that so many other reviewers are overlooking all of the cool new stuff just because it isn't 100% new rules. This is a fantastic product.
First off, most of the other reviewers are exaggerating how much of the book is reprints from other sources. James Jacobs has said that no more than 1/3rd of the book comes from other sources, but I agree with Rysky's review in that it's probably more like 1/4th. I also don't see it as a problem, as printing rules in a hardcover book makes it much more accessible to most of the playerbase. Not everyone has the money to buy every Player's Companion, and it also makes those rules easier to find, rather than forcing you to go hunting through your many spatbooks.
As to the new content, this book has a bunch of cool stuff!
The Eagle Knights got the Golden Legionnaire, a prestige class that seems designed to fulfill the stereotypical "tank" niche found in MMOs - it's built to take aggro and incentivize enemies to attack you instead of your allies. To date, this has been a character concept that has been very difficult to build a viable character for, but this prestige class goes a long way towards making it possible. I definitely plan on making a character to see if this prestige class is as useful in practice as it seems on paper.
The Grey Maidens got showered with love in this book. It seems like everything they got is an option PCs can seriously consider taking, as opposed to just being something the GMs can use to add flavor to NPCs. Rysky's review covers the Grey Maidens pretty thoroughly, so I'll just refer you there for the sake of time.
The Houses of Perfection has the Student of Perfection prestige class. Monks are my go-to class, and any time they get an option worth considering I find myself practically leaping out the windows with joy. This prestige class is available to non-monks, but it's pretty obviously built to let Monks (and Brawlers, to a lesser extent) take levels in it without actively making them worse.
Like the Golden Legionnaire, I'll have to actually play this to get a better reading on how good it is, but I have hopes. (At the very least, it's not worse than an unarchetyped chained Monk, although I realize that's not saying much).
If the Student of Perfection is the Monk prestige class, the Rivethun Emissary is the Shaman prestige class. My attention tends to gravitate towards martial options over spellcaster options, so I'm not really an expert on this topic, but at first glance this looks like a pretty reasonable prestige class. It's not out-of-control broken, but it looks like the options it gives you are solid and at the very least not awful. Like I said, I'm not an expert on spellcasters, but this prestige class seems worth looking at for your shaman-esque characters.
I only mentioned four of the 30 organizations in this book, but they all have interesting options and lore associated with them. Not all of them are viable for your average home game (the Mammoth Rider prestige class isn't technically bad, but it increases the size of your mount to Huge, which although cool and awesome and all that, has limited application in most games), but there's definitely something in here for everyone.
5/5 product, and I hope to see other hardcover books that follow in it's footsteps.
It'd be hard for me to say what I like about this book better than the other reviewers, so I will point you to the other reviews for this product for in-depth descriptions of what this book contains. I just want to let it be known that the Skinwalker is a great, versatile race that can do almost anything a PC is looking for, in one way or another. It's not overpowered by any means, there are very clearly-outlined fluff- and crunch-based drawbacks for having this versatility, but they only serve to enhance the race as a whole. Along with the race come various flavor-full traits and other options (feats, Magus Arcana, etc...) for PCs to use.
If you're looking for a Player's Companion to buy, I heartily recommend the Blood of the Moon.