I also use an iPhone on T-mobile. As jrmg said, you'll only get Edge since the 3G frequencies are different, but the speed is fine for most use. I also highly recommend T-mobile overall. Let's just hope something stops AT&T's buyout.
As someone who has used the iPhone on both, not having certain features enabled as a result were annoying as well. Such as visual voicemail (as one example). Sure you can live with it but its still an annoyance.
