I have Verizon 3G. On the TransAm, I had cell service but no data service in eastern Kentucky. I had no service at all in the Mount Rogers National Recreation Area, in Guffey Colorado, from Rawlins to Lander and in Dubois in Wyoming, in much of Idaho, and in parts of eastern Oregon and the coast of Oregon. On the Northern Tier, I had no service in parts of eastern Washington, western Montana, parts of North Dakota, the Adirondacks and central New Hampshire. Service is generally unavailable in most National Parks. Service was available, but I was unwilling to pay extra for it, in Alberta and Ontario (the only service that is included in your plan in Canada is text). Other than that, service was good. I could usually post my journal entry directly from my tent at night. Even in most of the dead zones I mentioned, I could still get a period of coverage sometime in the middle of that day or the next. From a data standpoint, eastern Kentucky and Ontario were the only place I went days in a row without coverage. You can sometimes find Wi-Fi to substitute when cell service is unavailable, although the available Wi-Fi can be so slow as to make it unusable for many purposes.
I also believe that Verizon is the best in small town America. If you're using your phone almost exclusively in a big city, other carriers may be better, but this doesn't usually apply to touring.