I've been very happy with AT&T. I started out with I think US Cellular, which turned into Cingular, which turned into AT&T. I've always had excellent customer service both in person and on the phone.
I live out in the sticks and coverage out here has been sketchy, especially at first, though it got better and better with closer antennas and better phones. This year I asked for and received a FREE microcell so I now have 4-5 bars at home as long as my wifi is working (Windstream).
Driving through rural Florida, I occasionally find a dead spot, but it's not often, and I can mostly stay in 3G wherever I travel.