I have Mint Mobile which uses T-Mobile's towers. I have no complaints with the service. I live in a fairly rural area in Kentucky and I don't notice myself losing signal any more than I did with Verizon near where I live and work. So for my usage it's great and saves me a ton of money.
Last month I took a trip to Colorado and I can confidently say that T-Mobile is not the best in desolate areas, like those long stretches of interstate between towns in central/western Kansas. My wife's phone uses AT&T towers, and she had service in a lot of remote places that I didn't. But for the amount of money I'm saving, I can certainly live with it, and I think T-Mobile service overall is improving quickly.