Depends on where you live. Most places I've been T-Mobile's coverage has been excellent. In my home town they actually have better coverage than even Verizon. The only exception I would have to give is travelling to remote areas of the southeast, where T-Mobile often doesn't have any coverage at all outside of the biggest cities.
As I was driving across the west/southwest a few months ago I was monitoring coverage and speeds on T-Mobile, AT&T, and Verizon as I drove. AT&T's coverage was by far the worst of the three... quite spotty with ton of dropped calls. Verizon had coverage for nearly my entire drive -- only lost signal a couple of times, but LTE was only available in limited areas, and their 3G isn't very fast.
In most cities of any size, actual throughput on T-Mobile's "4G" was at least as good as, and in the majority of cases faster, than AT&T's LTE -- T-Mobile consistently got 8-13Mbps everywhere I tested, but AT&T, even in LTE areas, was all over the map. I never saw a speed on AT&T LTE over 13 Mbps, and most of the time it was more in the 6-10 Mbps range for LTE and 2-3 Mbps for 3G. Verizon was by far the most consistent in terms of coverage. And their speed was consistent for a given cellular technology -- 10-18 Mbps on LTE, 1.5-2 Mbps on 3G.
I know that AT&T LTE can be fast. There is an AT&T tower across the parking lot from my office, and I consistently get 13-15 Mbps when I'm at work. But they tend to space their towers farther apart than other carriers, so coverage quality varies a lot more with their network than the others..