I held out on buying the BoVD for a long time. Eventually the Dungeon adventure sold me on it and I bought it. Haven't been disappointed.
The book is very useful, for many of the reasons that people have mentioned.
The Prestige classes are very swappable and contain some interesting mechanics you could easily lift to "enhance a villian". The thralls could easily be adapted to other evil organizations (Graz'zt would be a great one for Thay in FR, for example)
With regard to the Demons, princes, etc: A lot of the political and the core aspects of the creatures can be used without actually throwing a CR 20 virtual god into your game.
Ex: Belal (5th level of hell, I think) could just as easily be a depraved mortal king, ruling through his puppet daughter.
Liviticus (painfully mispelled) could just be a powerful wizard/cleric/psion who has been trapped in an intert form and communicates through a choosen follower.
In other words there are a lot of great ideas to mine and throw into the game. Even the simpler magic items provide a lot of interesting roleplaying opportunities.