It may be Sprint. At my poor access location I was told by a local that Sprint was the best carrier at the south end of the lake. The Karma coverage looks a bit like that, but I can't tell for sure. The Verizon coverage map is fantasy and the AT&T map is not much better. None of them even work well for cell phones at my place until the leaves fall.
Back in the 90's Sprint *way* oversold their capacity. A friend couldn't complete a call anywhere during rush hour. Since then they have expanded, but they are still at the bottom of the list for coverage and capacity. Still, it only matters if they cover where you are and your call goes through.
The data rate issues are why they say "up to 5 Mbps". I recall reading the Verizon contract and technically they didn't guarantee it would even work... anywhere, anytime. If you sign up with them and it never works you paid for nothing.
My roommate has AT&T unlimited at $100 a month. I would never pay that much for a cell phone. He can't tether so what is the point? It is his main connection to the Internet, but we do have WISP at the house, so most of the time he doesn't need it. The WISP is unlimited because it is a small provider. There is another WISP in the area that has a contract with the same limitations as the cell companies. I never gave them serious consideration because of that.