I just switched from T-mobile to AT&T. I gave up service for quality. T-mobile was quick to fix any issue I had, and most of the time in the stores. When I needed something for the phone just about any Tmo store had what i needed. The down side..before 3G I had no problems with calls at all. When they put in 3g call quality dropped, along with the average number of bars. Now they have HSPA + (( tested to be the fast data currently)) and I cannot get a signal in my house, yet i am about 1/2 mile away from the tower.
With ATT if you want anything assistance, accessories, or upgrade (( which my GF just did)) your only real option is going to the website. The stores do not carry much of anything in stock and the sales reps know little about the phone. However, I have never dropped below 3 bars, and have yet to of found a dead zone.