I recommend checking your area with AT&T Coverage Viewer
. As expected, it tends to be on the optimistic side. Translation: "Best" (dark orange) = solid (very few, if any, drop outs), "Good" (orange) = should be okay but expect some dropped calls and/or drop outs, and "Moderate" (light orange) = avoid AT&T.
I have been AT&T customer in northern California since 2000, starting with AT&T Wireless TDMA (some Nokia candybar), then AT&T Wireless GSM (Sony Ericsson T616), then Cingular GSM (Sony Ericsson S710a), and finally AT&T Mobility GSM (iPhone). In terms of coverage, Verizon is the king. Still, ever since AT&T started migrating GSM towers to 850 MHz frequency, the coverage has gradually improved. In San Francisco, the coverage is generally solid, with some dead spots in southern San Francisco and parts of steep hill areas. The coverage is generally usable in underground BART and MUNI stations but not between stations. In South Bay/Peninsula, the coverage is also generally solid, although parts of Palo Alto and "hill cities" (e.g., Los Altos Hill, Hillsborough, Saratoga) are mixed.
I don't go to East Bay much, but in my experience, northern East Bay (Berkeley, Oakland) and Fremont are pretty solid. I have been told that "newer areas" such as parts of San Ramon and Castro Valley are pretty bad.