I'm a tmobile user - phone provided by work so I never deal with customer service. With that said, the coverage maps are quite different. I live near San Francisco and while tmobile coverage is good, ATT coverage is much better and there are some spotty places (like my house) with tmobile, but anybody who visits and has verizon or ATT have full bars.
You can also have data and voice active at the same time with ATT. Not the case for tmobile (this is a problem for me as in my line of work, I get pages via Email, but when there are problems, I'm on the phone so I don't get pages until the call ends). Ditto for those who tether their phones.
Lastly, I went to Houston last year on a bus trip and WOW. Tmobile coverage is amazing down there (probably carry over from the baby bell days). I don't think I ever saw anything below full bars which was odd.
So it would seem there may be a plus on both sides of the coin and I'm looking forward to seeing how things will improve. Before joining tmobile 2yrs ago, I was a long time ATT customer (9-10yrs).