I'm with T-Mobile and certainly could switch to prepaid. Unfortunately, prepaid tends not to offer the same types of features/services I get with postpaid. Not to mention I'm on a grandfathered postpaid plan with stuff that newer T-Mobile postpaid plans do not offer.
Prepaid is great if data is all your after, but I have ten lines for four people and I doubt AT&T prepaid is going to get me anything comparable to what I get postpaid with T-Mobile.