I've got Sprint service on a Palm Treo650. I've almost never lost a call from my end. It works in the Grand Canyon, Atlantic Beach, NC, Half Moon Bay, CA, Denver, Yosemite, Sedona, AZ, you name it. Sprint merged with or bought Nextel, so I guess they are very similar. My previous phone was a Moto StarTac.

I've had Sprint service for about 5 or 6 years and have no complaints other than their dismal customer (dis-)service. If you need technical help, you're on your own. I expect that's standard with all of them, though. Everyone else in my family has Cingular and their calls drop out frequently and they seem to get out of range from their system very easily (in Washington, DC) in spite of being in an area where you would expect excellent coverage.