I use Verizon and find it to be pretty good coverage wise. They do have one of the largest coverage areas of any provider and pretty good customer support. If however, you like to use lots of phone features or anything, 1) Verizon's phone selection is terrible in comparison to AT&T's (Cingular) and 2) Verizon is notorious about locking out built in features like bluetooth file transfer so you have to pay them for their add-on feature. It seems like I can never win with the cellular companies, my honest feeling is they're all terrible.
I haven't used the new AT&T network but I had the old AT&T Wireless network several years back and it was pretty bad. From my experience, I've found Verizon to be the least terrible of the cell carriers.
"There are many dying children out there whose last wish is to meet me" -David Hasselhoff