If you haven't picked up the g1 and on the fence about it....I strongly suggest getting one, coming from an iphone user! The g1 has such amazing utility that blows every other phone out of the water. The platform has all the great features the iphone lacks: mms, copy/past, and just basic interaction between the applications that the apple totally missed the boat on. And let's not forget...the open-ness (if that's even a word) of the platform giving access to anyone to write whatever application they see fit and see a niche for.
Before buying the g1, I had some misconceptions about it: i thought i was going to be a joke with it's screen swipes and basic scrolling....Which it WASN'T. It was smooth and responsive, maybe a fraction of a second behind the iphone interface but you'll never know the difference. And I also thought, the swing out keyboard of the g1 wasn't going to compete with the iphone key board...I was wrong again. The full key board makes typing at least twice as fast.
Overall...I say get the g1, if on tmobile or not, I've heard people are already hacking them and using them with alternate carriers so there's no reason not to.
Cya...I hope this helped.