I moved from Sprint to AT&T in October of 2007. The coverage for AT&T has been very good. I travel to/through rural areas up and down the East Coast and while reception can be understandably spotty, overall I'm satisfied. In urban areas or along interstates reception is typically very good.
I live in a suburban/urban area. With Sprint and a relatively new phone, I couldn't get reception in my driveway or standing out in the street in front of my house. Sprint's attitude/service was horrible or non-existant. I'll never go back.
One thing with AT&T, make sure you understand the rebates/plans, although I think that's true with any provider.
Good luck.