The Nokia Lumia 800 is a great phone, but when it was first released it was a bit rough around the edges. Fortunately, the Lumia 900 is basically a suped-up, more refined, bad-ass version of the 800 that takes advantage of AT&T's super speedy up and coming LTE network.
Check out the full review of the Nokia Lumia 900 here:
http://www.chipchick.com/2012/04/nokia-lumia-900-review.html