Like everything Paizo has been doing lately this is a high quality product. The art is awesome, the layout is superb, and the content is top notch.
A staple of D&D systems is supplements with options for players to customize their characters. Every edition of the game has had a slew of such books. The quality has generally been highly variable. Most books have one or two neat new ideas in them, and a bunch of weird crappy ideas that no one ever uses. I have shelves full of past "Complete Warriors" and "Players Handbook 14's" which have seen very little use in campaigns I've run. This is the first such supplement I've ever purchased in which practically every page has a juicy new idea that I can immediately visualize being used in a game by one of my players in the immediate future.
My favorite thing in the book is the Prestige Class "Master Chymist". Designed to be a route for Alchemist characters to develop it is a wonderful riff on Dr. Jekyll and Mr. Hyde. The Master Chymist functions as an alchemist preparing extracts, tossing bombs, and whipping up magical potions to boost his allies most of the time. But this alchemist has imbibed a few too many of his own mutagenic concoctions. Several times a day he mutates into his alter ego, a bestial humanoid with a different name, and personality and goals - and supernatural abilities. When you build your Master Chymist you must design the two separate personalities (required to have differing alignments) and choose the particular mutations of your alter ego, which may be animal traits or magical resistances, strength, speed, sharp teeth or claws and such.
I already plan to create the pompous and calculating Lawful Evil Dr. Finias - investigator of the bizarre and macabre, and unscrupulous purveyor of hazardous chemicals; and his uncouthe mutagenic alter-ego "Moog" - Chaotic Good party animal and super-cuddly defender of people who feed him sweets.
