I'm very happy with AT&T. I've been with them for almost two years, and I've only had four dropped calls. All occurred when using my iPhone 4, never had one with my 3G.
My family was with Verizon before we switched so I could get an iPhone, and they sucked big time. We routinely had outages in our area that would only affect my phone and my grandfather's phone, and would leave my mother's phone and my grandmother's phone working perfectly. We all had the same model of phone. I could literally stand right next to my mom, try to make a call, and it wouldn't connect, but if she tried on her phone there would be no problems. My grandfather who is a contractor lost a job because clients were unable to get ahold of him while he was on the road, because of one of the outages that we had. Verizon basically told us to shove our phones, and deal with it.
Anytime we've had even the slightest problem with AT&T, my mom calls, and they'll credit our account with several hundred minutes, no matter how small the problem. AT&T sucks up to their customers, which is good business IMO.
-Don