I use T-Mobile and have for the last several years. I have found T-Mobile's coverage maps to be pretty accurate, at least for some of the mountaineous areas in the Front Range. There are several particular places where I know what my phone's signal strength is (or where there is no service) and that agrees with what TM shows when I zoom in.
http://www.t-mobile.com/coverage/
When roaming, GPRS has worked for me on AT&T and on USA 430(?), IIRC. It did not work a few weeks ago when I was roaming via Sprint (which surprised me that I was able to roam on a Sprint network at all since I did not think Sprint and T-Mo were using the same technology; shows how much I have been keeping up).