Went through all of this a year ago when I moved up here from Raleigh. Alltel is good from Grassy Creek to Clarksville but very spotty in Nutbush. Us Cellular is good for the lake but does not have good national service, roaming, etc.
I went with Verizon even though I had to go to Raleigh to get signed up. They gave me a local number, and coverage is excellent on 90+% of the lake. Its gotten to the point that I just forward my home phone to the cell for days at a time.
I've used US Cellular all the way from the Canadian border to Key West Florida. All calls were local as long as I stay east of central TN. Service and price are great.
Alltel is spotty at Henderson point area but Verizon works great in that area. They even have good service for Broadband/National acess for my computer using the Kyocera KPC650 wireless PC card. Service is not cheap but beat having to pay for installation of a satellite. The speed is about the same as satellite and the monthly rate are equivalent.