I live in the Boston area and work west of the city. While Tmobile is very good in Boston proper, it becomes problematic outside of the 128 loop. Service is also very sporadic in Newton and Brookline. While I have had Tmobile in the past, both my wife and I now have Cingular and service throughout the area has been excellent. As always, YMMV depending on where you travel.
z10 on AT&T (plus an HTC One X)