In southwest Florida, T-Moble is the worst provider there is.
Dead zones, dropped calls, and just horrible customer assistance.
Despite having dead zones along every route into town, we're pretty happy with T-Mobile in Redmond WA. Looking at coverage maps they're probably not the provider you want if you're in rural areas a lot, but my experience in cities (outside of Norfolk VA Little Creek Navy Base) has been very good.
Customer assistance is generally pretty good too, but then we're "blue"-level customers (top-rated long-time customers). Years ago the wife tried AT&T here... it was horrible.
I agree...T-Mobile is HORRIBLE here in West Texas. I switched to Cingular and am happy with their service.
Just in case someone invokes (invents?) an arcane CZJ Rule.
Coverage on all the carriers is far from perfect. If you live in a place with sucky coverage from your carrier, ask a friend using another carrier to come over and see if his phone works there. If it does, switch carriers.
For me, in my locations, Verizon has been great.