Hey
im not on t-mo with my iphone, but was with my previous phone and one of the reasons for leaving was the deteriorationg gprs service, speed and availability. I was finding I had several hours a week when I couldnt use my phone for data - and being obsessed with my email that wasnt good for me.
Worryingly I couldnt get t-mobile to even admit to a problem, and I know it wasnt just me or my phone, swapped with my daughter who had a similar phone on t-mo web n walk, and hers was the same, and my work mobile suffered the same problems. The unofficial word from someone who knows but probably shouldnt say is that the popularity or web and walk has outstripped the capacity a bit.
Therefore i left, nothing to do with O2 having the iphone of course