T-Mobile announced that they are removing the already industry-leading 6 month wait between phone upgrades and are now letting users upgrade whenever they want. The fine print is that you're limited to 3 phones per year, but that's still pretty insane. I have taken advantage of T-Mobile's JUMP program extensively, having owned 4 different phones in the past 18 months. They're not that much cheaper than other carriers and their coverage is pretty poor, but the fact that I can upgrade my phone whenever something new comes out is keeping me with them for the time being.
I've talked to a some people in the mobile business outside T-Mobile and they're gobsmacked when I tell them what T-Mobile lets me do and how much I pay per month. They're convinced T-Mobile is losing a ton of money on me, and I'm sure they're right. I'm an anomoly (I write a tech blog for god's sake), but T-Mobile is really playing to people like me, and I like it!