I Wouldn't Say Reliability Tops My List
When I tried AT&T three years ago, it was truly horrible in SE Michigan. Holes everywhere in coverage. That has improved quite a bit and I know of only one coverage gap in my travels.
Reliability isn't first on my list though. What might keep me on AT&T versus Verizon (where I know the network is far better) is the fact that I can get good unlocked GSM phones for $50. And I can get great phones that are not wounded by Verizon's paranoia and greed.
The lesson here isn't for AT&T, the lesson is for Verizon. Give the customer what they want and stay out of the way.