The whole book has alot of great information on dragons. One of the things i've found most useful though is the random art object generation tables. Need 30-some art objects for youre dragon's hoard? No problem.
In my opinion, there were like 4 different attempts to cover Dragons in a single book and all pretty much failed until WotC released "Draconomicon". It by far was the most complete book of them all, where some might say they wanted only parts of it, look at what else these same people are saying, there was so much it appealed to almost everyone. I usually have AEG and MGP's Dragon's books close at hand but honestly now I never even bother to open them. AEG sometimes will get opened and MGP, I personally have stopped buying any of their material just because it never seems complete. I am still kicking myself for buying City of Sheoloth.
Anyways... Draconomicon is an extremely complete book and if you are looking for secondary material to enhance that, a few of the Dragon issues have provided such material. If your world deals with dragons, trust me when I say it will assist you in creating something very unique for your players to experience. Oh and for your players, please come into the dragon's liar with their coins seperated by type, it just makes it easier on the dragon to clean up afterwards.
I think everyone has already expressed my views. Great artwork, sample dragons and lairs, great prestige classes, and new feats & spells for dragons. Its very well written and is a great read as well as being a top notch sourcebook.