It really depends on where you live and plan on using your phone. I have T-Mobile and it works great in the Atlanta area. I live on the far north eastern edges of the Atlanta Metro area (technically slightly outside the Metro area) and my phone works just fine while I am outside. However, once I go inside (including inside my house) I lose signal.
On the positive side, T-Mobile allows your phone to work seamlessly over wi-fi. Just beware, if you start a call on wi-fi and leave, the call will not automatically switch to the cellular network. The same goes for a call starting on the cellular network and trying to transfer to a wireless network.
If you travel a lot, I would suggest checking their coverage map as it is not very good, especially when you look at more rural areas of the country.