This is a pretty good sourcebook. Here are some pros and cons:
Pro: There is a fantastic selection of mutations in here, way more fun than the original book. Excellent additions also in character descriptions and foci, and some provocative new creatures. Fun new cyphers and oddities, very weird ones. By way of example, my players were both annoyed and pleased with the antler that you stab into yourself to become poisonous jelly that heals back lost limbs.
Con: Rusthaven itself is cool, but feels like a fairly empty place. Only one really neat feature to it. The adventure that comes with the book is good, but kind of falls apart at the end as there is really only one choice for the characters to make and not much in the way of puzzles to figure out.
The art is sometimes great, sometimes so so. Overall, I found the pdf version of this to be a good buy.
As soon as I bought this sourcebook, I began to integrate Rusthaven into my campaign, so that should say that I do recommend it.