I've been with T-Mobile since they were one2one, and I can prove it because my phone still says "one2one" on the screen.
(People keep telling me it's something wrong with my phone, but I've changed phones several times and every time I switch a new one on up pops "one2one" like I've got some kind of crypto-time-travelling SIM card.)
I'm not very likely to move since I get half-price line rental, but their coverage seems very good (at least where I live) and I never seem to have problems making calls.