T-Mobile really needs to improve their coverage. They're speeds are already good but their lack of coverage is the one and only reason why I can't even consider them.
This is where a little tech info helps. Look at page 7 of this T-Mobile FCC document about the future 600Mhz spectrum auction: http://apps.fcc.gov/ecfs/document/view?id=7022116151
Because Verizon and AT&T have almost all of the "Below 1Ghz" spectrum, it makes it far easier for them to build out rural networks. It's not that T-Mobile doesn't want to compete in those areas, but it would be too costly to do so with the AWS and 1900Mhz spectrum licenses they have. To cover the same area, they'd need 3x the number of towers!