As part of my job, I carried an AT&T cell phone, a Sprint Aircard, and a Verizon smartphone. I have taken them as far East as Boston, North to the Lake of the Woods in Minnesota, West to the Rockies, and South to the Grand Canyon. There were a few spots where nothing would work. But almost all the time I had Verizon coverage. Sprint would frequently work (not as good as verizon though). AT&T was the worst - OK around most of the large cities but frequently would not work when Verizon and Sprint would.
Now I carry just Verizon and have been happy with the coverage (expensive, but worth it).
