Switched from AT&T to T-Mobile, once the 6s came out. I mainly had three reasons for switching:
1. AT&T would not allow me to use my mobile hot-spot, because I had a grandfathered unlimited plan
2. I was paying about $50 more every month (counting $40 more in standard fees and ~$10 in occasional Canada calls).
3. AT&T customer service seems to be getting worse... treated me like poop, and were always trying to get me to drop my unlimited plan. Seriously, if I called them about a tech issue, they'd turn it into a pitch for me to drop my unlimited plan
Anyhow, my big concern was with the network. So far, my experience with that has been OK. I've never lost voice coverage when I needed it, though I will admit that sometimes my data speeds seem worse, though sometimes it's screaming fast. The most disconcerting thing is that I now have lots of situations where I only have one bar, which initially worried the heck out of me. But I don't know whether it's a T-Mobile thing or a 6s thing, but even 1 bar doesn't seem that bad. Overall, I'm feeling like my concerns about T-Mobile's network were overblown. But time and more travel will tell?
So far, so good. The mobile hot spot capability is amazing! I use it often and don't know how I lived without it, for so long.
Weird thing is that I would have stayed with AT&T, even though they were charging me considerably more than T-Mobile does, if they just would have let me use the hot spot capability. But they didn't, so AT&T wireless lost a 20 year customer relationship. An that's what I don't get - they pay a ton of money on Marketing trying to attract new customers, while treating their existing customers badly. Oh well...