Well, I read through much of this book last night, and I have to say I really liked it. It's not all applicable to all my games, since I use a much more relative alignment system in many of my campaigns. (That is, good and evil are
not active powers or absolutes, but simple definitions of action, and spells like
detect *alignment* don't function.) However, for those games I run/play where I do use the alignment system as written, the book gives a lot of interesting insight into how to portray "uber-good" characters. And good chunks of it--including most of the mechanics--are viable even for more relative-alignment campaigns.
In fact, my only real objection--and it's one that's easily ignored--is that it gives mechanical systems for converting evil creatures to good. One is a spell, one is a long, drawn-out series of rolls and saves made when a good character has many days to converse with an evil prisoner. I'm not a fan of any effect that forces alignment conversion or similar
permanent changes in personality. I feel the conversion of an evil creature really needs to fall into the realm of roleplaying and DM fiat, with the dice taking no part whatsoever. That said, that's a very minor gripe, since it's maybe four total paragraphs out of the book.
All in all, I think I'll actually get substantially more use out of this one than I did the BoVD. And it's actually more mature in some ways, in the true sense of the word--discussing potentially sensitive issues in a calm and rational manner. If this represents the direction WotC wants to take its "Mature Line," more power to them. I'll be there, credit card in hand.
I also really wouldn't mind seeing a book of this sort for neutrality--an idea I'd utterly dismissed until recently. I think it'd probably have to be shorter, maybe a softback, but it would be interesting to see.