I'm using an unlocked 4S on T-Mobile and to be honest, it is quite disappointing. I knew this coming into it, but I didn't want to write T-Mobile off without giving it a try. I switched from AT&T for the unlimited data and lower price. When I do get 4G, its great but usually switches to Edge. Getting 4G indoors is not common either. I'm contemplating trying an AWS capable phone on T-Mobile, but don't know if I'll do that because I don't want to pay a restocking fee on equipment. I'm in Albuquerque but have been to Baltimore, DC, and Dallas with disappointing results.
I'll probably switch back to AT&T when the next iPhone is out.