I've been a T-Mobile customer for years. Coverage here is not an issue for me (YMMV, actual mileage may be different. See dealer for details). Wife and daughter are also on T-Mobile.

I travel all over the world. T-Mobile, IMHO, has the best global roaming agreements. I do benefit from having the 850 MHz band when it roams in places where T-Mobile does not have a network, like the US Virgin Islands.

The number one reason I stay with T-Mobile is that we get superb customer service from them, both in the US and International. I can't get the time of day from Cingular, and I work for a company that has over 1000 BlackBerrys and several thousand cell phones.