Being part of a relatively conservative gaming group, the only time I get to use supplements like this is when I'm GMing, so I guess I come at this from a slightly different perspective than a player.
The bulk of this book is extraordinary, providing players with great insight into what makes Golarion's evil religions tick. The section on Zon-Kuthon still gives me the shivers! The sections on the minor evil deities, archdevils, demon lords, horsemen and great old ones/outer gods are also great (if cut short for space reasons).
The parts that I was less impressed with were the feats and many of the traits--they seemed on the whole to be relatively unimaginative and to provide underwhelming benefits. The spells and such are fine.
In the end, if you're after flavour and an insight into how the evil religions of Golarion function, go get this right now. If you're just after mechanical things like feats, you might find yourself a little less excited, but overall this is a great addition to anybody's Golarion library.