My current service provider AT&T Wireless for my primary/business mobile phone. I can and do use, depending on the known coverage and need for data, a couple of different phones... In areas of good GSM coverage, I use a Nokia 7210 or a Nokia 3650. In "iffy" areas, I carry a Multi-Band Siemens S46, which works on both AT&T's newer GSM network and the TDMA (aka, the older "Digital" network.)
I travel nationwide, and am quite happy with my AT&T Wireless GSM (and TDMA) coverage. I travel in California (SF to San Diego), Ohio (Dayton to Columbus), Michigan (Detroit metro area), NewYork (Manhattan to the Bronx to LI), New Jersey and Philadelphia (form Philly to Atlantic City), Maryland (Baltimore down to DC), Florida (Palm Beach down to South Beach), Texas (Houston/Dallas/San Antonio), and Arizona (Scottsdale), Las Vegas (though coverage deep in the bowels of the casinos gets sketchy).