Just got off the phone with T-Mobile after having problems with my third Motorola T720i. The first randomly crashed while in the middle of a call, the second also crashed and its external screen backlight didn't work, and now the third's buttons don't work. Lo and behold, they have a lemon policy where they'll switch you out to a completely different, new phone after three swaps, so in the mail I have coming a new Motorola V300.
So the moral of the story here? Stay away from Motorola T720 and T722 phones, but T-Mobile is great.