I have T-Mobile. Chose them at the time because it was the cheapest way to get into a family plan and get a couple of phones...total actual out-of-pocket costs ran $39...got two Nokia 3900 series phones. Dependable and few frills...but I want a phone, not a TV/PC/Video Arcade/Camera!
I made the switch because my Sprint phone was giving me problems and eventually, didn't work in most areas...kinda worthless to pay for a phone if you can't use it. But it seemed that when i tried to upgrade services or get different phones, that the charges were ridiculous!
All of the incentives were geared to new customers only. So the pitch to the loyal customers was "screw you, we already got your account!"
I am watching the number portability issue as well. T-Mobile (and a few others) plan to absorbs the tech costs for now, but the rest of you with other carriers, watch your bills closely!