Pros: Great coverage in the SF bay area
Cons: They done me wrong
After a few maddening interactions with the customer service people at AT&T back in 2005, I switched over to T-Mobile, which immediately impressed me with its customer service--it was night and day. At one point a rep. even offered to call me back in a month to check in on a change to my account.
Unfortunately, when it came to doing what I was actually paying them for--that is, making it possible for me to talk to other people not within shouting distance--they couldn't hold a candle to AT&T, at least not in SF. Dropped calls, dead zones, etc. And when, just after a month in, it became clear that it wasn't an improvement at all, customer service turned hard-nosed, and refused to release me from my contract.
When my contract ended a year later, I promptly switched back to AT&T and managed to avoid talking to customer service for almost a year. Recent interactions have been better, but there have still been hiccups. A month after canceling my data plan, having explicitly told the rep that I had no use for it, I was bombarded with per-kilobyte service charges thanks to some invisible background process running on my phone. Some but not all of the charges were reversed, and while the rep I talked to was polite, it brought back bad memories.
All in all, it seems to me that customer service at AT&T may be coming more into parity with its mobile service, which for me has been very satisfactory.