I'm probably going to switch to T-Mobile because of the faster HSPA+ speeds, and cheaper service but until then I'm keeping my AT&T service. Went to the store today, had them cut down my SIM card since the old one was too big. The guy (who was kind of a jerk) cut the sim, popped it in, rebooted the phone and sent me on my way. Everything works, but like many others, data is WAY slow.
If I understand correctly, changing the APN won't help much unless you have the proper plan. My account shows that I'm still on a data plan for an iPhone 3G. I wish there were a way to do this myself but it looks like I will have to go back (or call) and get on a 4G data plan first. Maybe it doesn't matter since I'm switching, but I'd like to have it working properly in the mean time...