I've been with T-Mobile for about three years now -- switched from Cingular when I realized that my other BB, the 7100g, didn't get a signal in either my college area or where my family lives. Except for that little detail (freaking Cingular...) I really liked it, but when I made the switch I went to a trendy pink Razr, then a Sidekick 3, and now here I am with the BB Pearl!
It's great to be back on a BB -- this time, with way better coverage. And I have always had good experiences with T-Mobile Customer Care and all the stores, which is something that always affects my decision to stay with a company. The plans are nicely priced, too.