I ditched Verizon to get an iPhone from AT&T, but can't really compare the two services, as my iPhone is my first foray into data on a handheld device. However, getting away from Verizon's god-awful customer service was fantastic.
Realistically, I couldn't switch back anyway, as I've still got another year on my AT&T contract, and am in no mood to shell out extra cash for cancellation fees, new activation fees, and another iPhone. In any event, I'm perfectly happy with AT&T's service for the most part. I'd like it if I could tether my iPhone, but all things considered, I've never had any issues with my AT&T coverage that have caused me to consider a different carrier.
I am considering ditching my iPhone altogether for a Droid, though, but that's another story entirely.