Actually, T-Mobile's service is pretty good ... a site better than Sprint's and the data speeds are faster than Verizon and AT&T. Sure, there are coverage gaps but I think T-Mobile is really beginning to shore those up with the purchase of some 700mhz spectrum from Verizon and the announced efforts to upgrade areas on 2G to LTE, T-Mobile may rapidly become a force to be reckoned with. Wall Street and consumer-America seem to think so as they have had a whopping number net customer additions.
You also miss the point that a contract in the wireless telecommunications industry heavily favors the carrier and, if you are under contract, you cannot just dump the service because it starts to go downhill. Well, okay, you can by paying a hefty early termination fee. Having contract free wireless forces carriers to actually provide a highly reliable, good quality service. If they do not, the customer simply drops the carrier for one that is.