T-mobile customer service was some of the best I've ever had. I usually think about customer service like I do umpires - when they do right, I don't think about it and when they do wrong, I hate them. So it takes excellent service for me to say, "wow this is great service" and that's what I got with T-mobile. I haven't had to call in a long time, but back when I did, I remember thinking it was great that the reps I spoke with were polite, well-spoken, eager to help, knowledgeable, and empowered to do something about my issues. (I've read that their customer service ratings have ebbed somewhat lately - not sure what that means in practice though).
But I'm in a hinterland area, which for T-mobile means spotty coverage. If you travel to lots of random places for work, for example, they're not the best choice. If you're in a significantly peopled area or along the corridors in between them you're fine.
I'd be a bit wary of going with either T-mobile or AT&T as this merger progresses. That may not be rational, but it just makes me think of headaches to come.