I've got Verizon. Good signal and coverage overall. The only time I lose it is if I'm in someone's basement or something like that. I have never lost it on my trips to and from Atlanta or my trips to the rural NC mountains or to the coast. I think Verizon has the eastern portion of the nation pretty well covered.
Not sure if this is important to you or not but Verizon does restrict your freedom when it comes to how much you can modify your phone. I've got a Motorola T720 and I should be able to download stuff off of motorola's site but with Verizon I can only go through their "Get it Now" junk. Providers like T-Mobile provide more freedom as far as that is concerned.
The only service I would really recommend against is Alltel. One of my friends has it and get horrible service here in Raleigh. My uncle used to have it and got horrible service wherever he went it seems like. He would be driving down home from work and would hit holes in coverage where he would just drop conversations. Other times it just flat out could not get service. When we were in Alltel stadium in Jacksonville he couldn't get service... but my Verizon phone could. I found that quite amusing personally.
