I've had T-Mobile since June of 2005 and am very happy. Just renewed for two years when I bought my Curve last week. Coverage has been just fine and service has been great.
A while back, when I was going through a financial crunch and was short on cash, they sent a disconnect letter. I called and asked for an extension on my disconnect date of a few days to get to my next paycheck and the rep was only too happy to oblige and was extremely gracious and polite to me. It made me feel so good that I rearranged my bills and paid down my entire balance in full rather than just the past-due amount. Now that the money situation is much better, I'm only too happy to stay with them.