It's true AT&T blows. I had it before they had the iPhone and switched over to Tmobile because I had to stand by a window in my own house to talk.
I'm running iPhone on Tmobile right now...past 3 years since original iPhone came out. I don't have one complaint about it other than the battery life is terrible.
I use it primarily for email, web browsing, iPod, phone calls ...in that order, so phone reception isn't that important to me even though I'm very happy with Tmobile. The thing that makes iPhone so great is the community of developers constantly coming out with new hacks, apps, themes...etc on a daily basis for it, which to my knowledge no other phone comes close to.