I've now had the chance to read through the first five books in this adventure path (the sixth is yet to be released) and I must say I am deeply impressed by the quality of this AP. Each book is 100 pages, with approximately 80 pages devoted to the adventure, ten pages of fluff (a gazetteer or similar) and another ten pages of alternate rules (such as building your evil organization, feats for vampire and lich PCs, etc).
As for the adventure, it's very well done. The characters are interesting and all the major NPCs have proper back stories to them or enough hooks that it's not difficult to build upon. The first couple of books are fairly linear in nature (as are many APs, to be honest) and it may take a little skill on the DM's part to avoid a feeling of being railroaded (or players who aren't overly concerned about it). But the tracks are there for a reason and the expectations are set out fairly clearly at the beginning that PCs need to be of a particular temperament; by the fourth book, the tracks begin to branch and players will have a great deal more flexibility. But, even with the "rails", reading some of the PbP games here on the forums will reveal how creative players can be in terms of coming up with different solutions and approaches to problems and challenges.
There are moments of humor and levity mixed in, which is (I think) important given that this is supposed to be a campaign about having fun being evil (not despairing over your cursed fate).
The artwork is also quite good -- almost all full color and well drawn. I particularly appreciated the landscapes of locales in which the party would be adventuring (or castles they'd be assaulting/breaking out of/etc).
In short, if you're looking for a change of pace in your APs, give this one a try. Fire Mountain Games is forcing me to reconsider an old bias of mine against third-party publishers. This is an *excellent* AP that deserves any serious GM's attention.
