I've been using T-Mobile for several years now and have no problem with it. I switched from Verizon (I had an issue where Verizon would often send calls directly to my voicemail and no one could figure out how to fix it - being on call, this was unacceptable).
I live in Raleigh and travel to Durham regularly and don't have any coverage issues. That said, I'm not a heavy phone user, so perhaps others have different opinions.
All that said, T-Mobile doesn't have contracts, so you could also pick up a sim and try it out for a month (this is particularly easy if you have AT&T and so are on GSM already). If it works, switch, if not, stop payment and throw the sim away.