There's no doubt that T-Mobile's network has greatly improved since Legere took over the reigns at the nation's fourth largest carrier. I've been on T-Mobile since just before Legere took over and I've seen huge changes in their network in my area in that short amount of time. Going from barely getting a signal, to basically a full signal with pretty decent speeds. Now that Sprint is trying to buy T-Mobile and merge the two so they can better compete with Verizon and AT&T. T-Mobile has already bought some low-band spectrum from Verizon earlier this year, that'd be the 700Mhz spectrum that will be used in their network later on this year (remember this stuff takes time, it can't be done over night as much as we wish it could).
Right now there's a lot of rumors going around about this T-Mobile/Sprint deal, and many still feel that it's not going to get approved. Unlike when AT&T was attempting to buy T-Mobile, T-Mobile isn't just sitting still waiting for the deal to be approved. They have a backup plan in motion. That backup plan is simple, buy low-band spectrum from smaller, competing carriers. The one thing that T-Mobile really needs is low-band spectrum. And the reason being is that it penetrates buildings much better than the higher bands, like the 2.5GHz that Sprint is using for Spark. The 700MHz block of spectrum they bought from Verizon earlier this year covers about half of the US. However, the other half is covered by about 33 other carriers. One of them being US Cellular. So it'll be a great task to get the rest of that spectrum for T-Mobile, but I think it can happen.
Then there's also the 600MHz spectrum auction happening next year. Which the FCC has changed the rules a bit to make it more fair for Sprint and T-Mobile. Oh and by the way, the current deal that's being rumored between those two includes a breakup fee of $2 billion. I wouldn't be surprised to see T-Mobile using that cash to buy more spectrum or work on their network some more. They have come a long way in the past two years, but still have a long way to go.