This really is one of the best adventures I've ever seen. Lots to do, plenty of hooks and NPCs, helpful hints and examples for creating a "weird" atmosphere, and winged apes! 5/5 and a 'must buy' award if you're a pulp/Lovecraft fan.
The two big planar trips are quite similar and the cool new planar stuff the players get to mess with only makes the "boss" stronger. Not fun! Also the final battle is a nightmare to run. Finally there are a lot of monsters you will need to sub out unless you have Tome of Horrors as the stat blocks are not complete, giving only HP, CR and a page reference.
There is a lot of nice ideas to expand the scope of the adventure though - kudos for this!
Great book that delivers a solid version of the cavalier, in both fantasy and D20 modern formats, plus loads of extras that are useful to characters of all classes, e.g. lots of mounted combat and leadership based stuff. Awesome book. Loses a star due to numerous references to other related products.