I switched from T-Mo to AT&T, primarily for the better network in my area, but also for the 3G. The browser on the Bold is still disappointingly slow, even over 3G. Not the network's fault - it's the BB browser that sucks. Opera Mini is pretty fast though. And receiving attachments in emails is wonderfully fast.
If you have good AT&T network coverage in your area, it's a definite good move.