I have my iPhone 3gs and an epic with sprint, and I honestly get better coverage with the epic wherever I am. I live in Baltimore, and work in columbus. I never liked sprint until now. They still offer unlimited data, and its only $50 a month.. well $60 with 4G.
I still have yet to use my at&t upgrade, and have thought about getting the i4, but I don't think I'd use it much more then I use my 3gs, which is not a whole lot.