I'm still on AT&T. I'm (for the time being) switching to Sprint because I travel back and forth to northeast Ohio frequently (mostly to see my family) and I have horrible service in my parents' house. There are two spots in the house where I can actually call and text, and neither of those spots is 100% reliable. As far as I'm concerned, Sprint's service can't possibly be any worse. Even if the service isn't much better, the Sprint plan is cheaper than my AT&T plan, plus I get unlimited data, so as far as I'm concerned I've got nothing to lose.
The question mark for me is the quality of Sprint's service in Harrisburg (where I live and work). If it's comparable to what I get with AT&T, then I'm porting my number and not looking back. If the service sucks, then I just return the phone and cancel the plan within 14 days.
I did this with Verizon and the Droid Bionic last year. I opted to stay with AT&T because I couldn't stand the screen on the Bionic and I wasn't keen on VZW's data plans. Hopefully things will work out this time. I'll end up getting hit with the ETF from AT&T, but I was hoping to just sell my iPhone 4S (64 GB) to offset that.