I just got the Player's Handbook Races - Dragonborn book. Read my full review here - I just started writing for the Eye of the Vortex game site, which I am excited about because I will be getting free stuff to review soon!
I play a dragonborn ranger in my weekly 4e game, and while I have been having fun with her, I knew next to nothing about how dragonborns fit into the default 4e world. After reading the book, I have to say that in many ways this book feels "old-school" to me. There is a ton of inspiration crammed into 32 pages, and without fully detailing anything, it creates a sketch of the ancient dragonborn kingdom of Arkhosia, dragonborn family and clan structure and dragonborn religion that made me envision running a game based on uncovering the past glories of the old dragonborn empire. I guess what I am saying is that it evokes rather than details.
In an odd way, I would almost recommend this book more to someone who didn't play 4e! To the 4e player, it is going to be a disappointment in terms of the amount of new "crunchy" feats, powers and items, and what it does have in that department is going to show up in D&D Insider anyway. But taken as a systemless inspiration for a campaign, it is actually pretty good!