T-Mobile has one thing that none of the other carriers seem to, which is
UMA support.
While I agree that they seem to have the smallest network footprint among the three or four "Major Carriers" in the US, the fact that I can use WiFi instead of the cellular network overcomes pretty much all of those concerns for me.
For example, when I'm home (in a rural part of Chester County, PA), my cell signal from T-Mobile is only around 1 bar (of 5), but as I have a WiFi network at home, it doesn't matter at all. Also, I travel overseas quite a bit, and the fact that a call from, say, France, is treated as a local call (so long as I'm connected via UMA) overcomes just about any reticence I'd have over using T-Mobile.
The real shame is that none of the other carriers seem to offer this option, and the majority of the T-Mobile phones no longer offer it either. I've stuck with my Blackberry mainly because of its UMA compatibility. If any of the Android phones (notably the Nexus One) or the iPhone offered this, I'd take it in a heartbeat.