A very flavourful product that will be useful to both NPC's and PC's of a diabolical bent. Information is presented on each of the levels of the Nine Hells, and the archdevils who rule them. There's also extended information on the hierarchy of devils, and some new LE fiends to unleash on your players. Add in some new spells and magical goodies, and this is a great package.
There's also information on literally dozens of powerful devils, from the arches on down, who accept worshippers - which can add some new devilish options to spice up an evil cult, or give you some new character concepts.
A fantastic product overall. Great art, easy layout, and detailed information on all the major deities. Plenty of flavour makes this almost a must-have book for anyone playing a cleric in Golarion.
The only reason I rate this a 4 instead of a 5 is that there could have been just a little bit more content, such as info on the Vudrani pantheon and belief in philosophies instead of particular deities.
Still, only a minor quibble. Definitely worth purchasing.