If I may be so bold as to offer a personal opinion: The Book of Madness and the Libris Mortis were both about twice as awesome as the Draconomicon, and in fewer pages. I for one am fairly certain that all that needs to be said about the dragons could be fit into one book of this size, or two lesser ones, if they absolutely must have twinned tomes for the chromatic and their shiney-assed cousins. This book is nearly a whole extra PHB. I find it rather disconcerting that a single type of monster is going to take up two entire books, and come near to double the player's handbook. Quite frankly, I suspect this is going to contain a hell of a lot of filler.
I'm not rabid about this, I may be proven wrong. But I'm pretty anti-filler. Not only is it a transparent tactic intended to justify higher prices, it also actually reduces the quality of the text as a whole - a really tight, well-edited book is more useful than one I have to wade through to find the gems.
