I was with Verizon for over 5 years and loved them. No dropped calls, awesome coverage, and excellent customer service. Unfortunately I bought into the iphone craze and HAD to switch to AT&T. Their customer service is absolutely the worse, both on the phone and in their stores, plenty of dropped calls, and crappy coverage unless you in a major city. However, no other US cell carrier has the international coverage that AT&T provides. Last month in Vietnam and This month in Poland and Bulgaria with awesome phone coverage. Actually get a better signal here than at home! If I switch jobs that don't require international travel I'll be back with Verizon in a heartbeat.
BTW - I like my iphone, but not AT&T