J.C.
I left Cingular for US Cellular, and I love it. They were the cheapest, and regionally the most popular when I researched it with Consumer Reports about a year ago. But Verizon was the nationwide favorite. T-Mobile was also well-rated, so you can't really go wrong with your choices.
The signal strength/availability is better with US Cellular than it was with Cingular. Although they are regional, they do have national pricing and service in I think all major areas. When we signed up they had a lot of family deals where you can get multiple phones. We got Razrs (I think they're ugly, but they work fine - the speakerphone is amazingly good quality for a cell phone) for a really good price, and we have a shared-minutes plan.