I like AT&T if just for their GSM service. Only AT&T and Tmobil are GSM in the USA. Verizon and Sprint are CDMA. The whole world is GSM, except those two in the USA.
I land in France, switch on my phone, and I'm connected with Orange, or BiTell, or F-SFR. I land in Italy and I'm connected with Wind or Vodafone. Automatic, without me having to tell ATT to switch anything on. If I decide the cost is too high, I just go to a local cell phone store (or on the Internet) and buy a local SIM card. Matter of fact, in Europe, I carry two phones. One with my American AT&T number, and the other with the cheaper European number. (must keep American number to receive emergency calls, from family or neighbors).
In addition, I am out in Roswell NM often, and trying to get data from Verizon is very hard. No Verizon there. Connecting with Verizon, I would sometimes get on a 3G network, but mostly on a 2G network. I had to go to McDonalds to use their WiFi. Well 18 months ago, AT&T purchased AllTel from Verizon, and now Roswell has great Voice and data coverage.
Everywhere I've been in the USA, and in Europe, my phones work great.
In the very near future, I hope to get two new iPhones, and keep one of my old phones for a total of three phones. All I have to decide is if I want the new Family Data Plan, or the present plans.