I have AT&T, because I prefer it. It works at my home. Anyone who comes to my neighborhood, if they don't have AT&T, cannot get reception. Don't know why that is. Now, my mom lives in Santa Fe, NM. When I go to see her, I have to stand in the street out front to get reception. There, only people with Verizon get reception. JUst RIGHT at her house, though.
As far as call QUALITY: Dh, dd, and I all have different AT&T phones. We can all hear everyone fine EXCEPT my mom/stepdad on their verizon phones. We have issues understanding them because everything sounds fuzzy. Might be because they use bluetooths a lot?
OH, and I must agree with a pp that verizon gave us terrible customer service consistently. AT&T is the opposite -- every time I've called the CSR's have been friendly and helpful.
ETA: We were actually with Cingular which is now part of AT&T. I was concerned about the customer service, but after the switch to AT&T it stayed great. We have been with Cingular/AT&T for 8 yrs now!!