I switched this week after being with AT&T for over 10 years. The fact that I could bring in my own phone (I had just gotten a Nexus 4 from Google which was about $100 cheaper than getting it through T-Mobile) was a factor. Also a better service plan for the same amount each month (AT&T = 450 min, 200MB, no texting vs. T-Mobile = unlimited min, 2.5GB, unlimited texting). And apparently T-Mobile has no problems with hot-spot tethering. The sales rep made a point of mentioning that.
But as I learned after the fact (in spite of what coverage maps showed) is that I live in a small black hole surrounded by H+ coverage, so data is 1-2Mb (if at all) instead of 13-14Mb a couple of blocks away (grrr). I'm on Wi-Fi at home, so not a problem, but AT&T definitely has coverage beat in my area. The real test will be when I visit family in western Mass in a couple of weeks.