Before dropping out of the corporate world I sold network testing data to T-Mobile. Before you switch you have to understand the technology a bit. T- Mobile operates on a platform called GSM(global standard for mobile), as does AT&T. All the other US carriers operate on CDMA (basically).
So what does that mean? OK, a GSM phone can only operate on a GSM network. In the past, T-Mobile was the only GSM carrier so T-Mobile worked only where they had coverage. There was no other GSM network to roam to. And that is a problem when you have a smaller network built on metro areas and interstate coverage. When AT&T converted to GSM in the mid/late 2000s they did not provide a roaming agreement to T-Mobile, but instead tried to buy the company, which was blocked by gov't. There is now a roaming agreement between them, but again, coverage will only be where there is either a T-Mobile or AT&T GSM network covered by the roaming agreement.
CDMA carriers are different. Sprint/Verizon/US Cellular/ Cellcom/etc., etc. can roam between each other, where there are agreements, but generally there is CDMA coverage nearly everywhere. So, CDMA coverage in rural areas is generally better than GSM because the company does not have to build out a network in a low usage/low revenue area, they just use another carrier's network. Here in Door County my Sprint phone uses both the Sprint and Cellcom network. T-Mobile has (until recently) had no place to roam to. In Door County AT&T coverage is poor, but T-Mobile is just non-exisiting.
The two technologies also have different characteristics. GSM is very high quality, but if it cannot find a good signal it just does not work. CDMA will attempt to connect on lower quality signals, which is why you get the "I can hear you but you can't hear me" problem. A GSM phone just won't make that connection.
I know I am getting way too verbose here, but the net is, if you live in a metro area, travel metro highways and metro airports you will like T-Mobile, it is a good network WHERE IT IS. But if you travel to the UP or other rural areas it is the least likely carrier to provide service.
Also, I know there are a couple of wireless guys on this board, so if they have differing info then I defer to them!