In my area, their online street-level coverage map is fairly accurate to what I get travelling around. http://compass.t-mobile.com
One of our ops folks is based out of Ft. Lauderdale and swears by the coverage her 7100t gets down there, but I honestly don't know how far south she travels.
FWIW, all of T-Mobile's North Carolina coverage comes from Cingular. Well, crap, I think it's now SunCom since Cingular took over SunCom in SE Virginia. Either way, Cingular/T-Mobile coverage in NC should be the same.