Just curious which BlackBerry you have switched to on AT&T? I've always heard that service quality is great on AT&T, but their CS isn't nearly as good as T-Mobile and they're also more expensive. I'm very happy with my Blackberry 8300 (Unlocked AT&T model) running on T-Mobile - you might consider the 8320 on T-Mobile if you're not already locked into a contract.
BTW: I was also a Nextel customer and left just after Sprint bought them. The billing went to he__ in a handbasket right after they took over, so I was happy to be rid of them. They were very uncooperative in helping me upgrade to a new phone when I had an accident and dropped mine in the hot tub. I only had like 1 or 2 months left on my contract. So stupid that they would treat a customer so close to account renewal so badly. As a result, I started looking around and found the much better deal at T-Mobile.
--Kidd