i think it begins with where you are... in my area Verizon is #1, although, i took the plunge and tried it, and to be honest, AT&T isn't bad at all, my signal is really quite good.
as far as customer service, so far, not so great tho. I'm actually currently waiting on the phone trying to deal with a text-messaging issue, but otherwise the bill is concise, price is pretty equivalent or lower than my old Verizon/blackberry bill.
so.. if you're into the technology, take the plunge!