I just switched to T-Mo three days ago, mostly for price ($10 to add a smartphone to a plan with data). They're facing two problems - they don't have the cash to build out a network like AT&T and VZ, and they use higher frequencies that don't travel far enough or penetrate buildings. It's a double negative.
Their HSPA+ speeds are on-par with VZ LTE in my area but they're way too limited in coverage. I'm sick of looking at their coverage maps and seeing a "new cell site" with a range of 1500 feet. But 90% of my time is spent in good 4G areas so I'll deal.