Looking for an actual review from someone who has the book and plays 4e? Look no further.
Overall impressions - The book is for DMs. The old "sprinkle in some player bits into every book to increase sales" approach appears to be gone in 4e, and I'm all for it. Being in 3 games, only one of which is at my house, I appreciate not having to lug my entire library around on the off chance I'll need part of it that night. The art is nice in my opinion, and the book is a good mix of fluff and crunch, old and new (though really, did the world need a purple dragon?)
Section by section breakdown:
Chapter 1, Dragon Lore:
Good but not great - I never picked up the 3e Draconomicon, so a lot of it may be re-printed. I especially enjoyed the Chromatic Dragons In Detail section, which does a nice job of breaking down temperment, terrain preference, and favorite hoard objects in detail. Lots of little bits that I can see myself building out into adventures.
Chapter 2, DM's guide to Dragons:
Some very solid stuff here - lots of adventure ideas, campaign ideas, briefly sketched out Dragon NPCs, lots of hoards, some traps (I like the one that turns anyone who attempts to steal from the hoard into solid gold, making them part of the hoard), 4 artifacts, and some dragon rituals.
Chapter 3, Dragon Lairs:
3 heroic, 3 paragon, and 3 epic dragon lairs, each which is basically a mini-adventure (think dungeon delve with a bit more). Nice little bits which can be fit into just about any campaign. The last epic one is a nice shout-out to Red Hand of Doom for those who enjoyed that adventure.
Chapter 4, New Monsters:
First, let's start with the big deals, the three new dragons -
Brown Dragons: These are nasty, nasty boys. The blinding sand attacks, combined with the extra damage when the dragon has combat advantage, make for a vicious synergy. While I'm a little annoyed that they stole the desert from my favorite dragons (the Blues), they make good desert predators, and the gourmand fluff is interesting, though a little odd in the desert. Overall, good dragon, though I think I'll call them Sand Dragons instead of Brown Dragons to make them a bit more intimidating.
Gray Dragons: I really like the fluff for these guys - pure predators, they live for the thrill of hunt. Unlike most other chromatics, they don't spend as much time in their lairs, and are more nomadic hunters. The petrification attacks are quite nasty as well. Very nice dragon, will definitely see use in my campaigns.
Purple Dragons: Gag me with a spoon. I don't like these guys at all - the underdark theme, the psychic damage, and let's not forget, the fact that they're PURPLE . . . not everyone who does psychic damage needs to be the same color as mind-flayers, people. Total loss from my view, though they may be great fits for someone else. Loser Dragon.
Other monsters:
Wyrmlings - if you wanted baby dragons, you have them. Not much to say, though the art makes them very cute and adorable looking little engines of destruction (oversized head and feet, etc).
Planar dragons - some good, some bad. I'm not sure about the whole idea of non-solo dragons, and they're all slightly off-center away from the basic idead of dragon. Meh. The Shadow Dragon and Fairy Dragons make nice returns though (Shadow Dragon = scary).
The rest - a lot of familiar faces show up again - abishai, dragonspawn, etc. Decent spread of monsters from level 1-29. Nothing you could make a whole campaign out of, but you can probably find some stuff you like.
Dragon Hall of Fame: this section is pure win. Consider it your "epic tier opponents to smack the life out of your party with" section. Ashardalon, Cyan Bloodbane, and a few other famous dragons get write-ups, as well as several new dragons with some good backstory, and of course, 4th edition's first fully statted out deity, the unholy level 35 terror that is Tiamat. This is by far my favorite section of the book, and I'm already batting around several high level campaign ideas to make use of it.
Finally, there's some substitute powers to vary your dragons up a bit, along with a couple monster templates for your monsters (Draconic and Dragon Guard).
All in all, it's a good read, and if you're planning on running a campaign or adventure that makes heavy use of dragons, I'd definitely recommend it. You can definitely live without it, but I don't think you'll regret picking it up. The Gray Dragons and Tiamat are my favorite portions, though there's lots of good stuff to go around.