If you tether with your G1 and go over a 10gb transfer quota, they will reduce you to edge speed for the rest of the billing cycle.
I mainly stick with T-Mobile because my entire family uses it, so we all get to call each other for free, but I'm not against moving to a new carrier if I can get better speeds and be allowed to tether.
EDIT: I thought the donut release was pretty good overall, I saw huge performance gains and also the UI has been cleaned up a lot. Though not much else seems terribly different.