I've had T-Mobile (well, first VoiceStream) for a few years now and been nothing but pleased with them.
When there was a weird bug with my voicemail when I first got my phone, I called customer service, was immediately transferred to a real human being, and we worked through the problem, which was fixed right there while I waited.
The fees are reasonable, although make sure if you want free nights that you specify that you want them. (Old VoiceStream plans had no nights designation -- minutes were minutes except on the weekends). I've been given 200 free minutes a month for 'loyalty' -- it's kinda groovy.
When I upgraded my phone, I got a $100 rebate as a real check, not just a crappy credit to my account.
I've gotten my roommate, his girlfriend, and two other friends of mine to use this company as well and they're all quite pleased.
*looks up at her post*
No, I do *not* work for T-mobile

I'm just a firm believer in giving praise where it's due and I'm very satisfied with this company.
'
just my many many cents