When I moved off to college, my parents moved to T-Mobile (because it was cheaper), but I wanted to stay with AT&T. So we split off. The school I was originally going to had a contract with AT&T that offered I think 15% and they had a cell tower on campus so it was going to be good. Anyways after school I did some work with a few tech companies that offered 20-25%. My current discount is 23%, but what's really nice is that I get discounts on accessories with no upgrade or activation fees. I can add and remove lines from my MobileShare as needed. If a friend from abroad visits the US and needs a number for say a month, I can add them without incurring more than the basic month access charge. Anyways after college I merged my parents into my AT&T account without telling them. They were on auto-pay so I don't think they noticed until a few years ago when they lost a SIM card and needed replacing that.
We never did have T-Mobile signal at home so I'm not sure how they survived on it when I was away at school. I mean there is very fast Internet (and very up-to-date WiFi) and there is still a landline. Anyways I may never switch to T-Mobile unless there a very compelling reason to do so. Verizon and Sprint are out of the question since they are still CDMA carriers.
EDIT~
Seriously, a picture is worth a thousand words:
I know it's not important to a lot of people, but that upload speed is faster than many people's landlines.