We use Verizon and only found a couple Nat. Parks that service was missing. Works in place you would never think possible.
I think it is almost universally agreed that Verizon has the best coverage area. That being said, not all the coverage is data friendly. You will not get 3G some places or 4G most places. I also believe that Virgin runs over the Sprint network, so you will only get service near major cities.
As John suggested, Verizon has about the best coverage overall. If you're considering a Virgin month to month account, comparing their coverage map with Verizon's, or even Sprint's, the network they use, is enlightening. Virgin subscribers do not have access to parent company Sprint's roaming partner towers, and that really shows in their limited coverage.
I mostly use high speed broadband, and travel quite a bit in the SW and NW states, and and have found Sprint to be very reliable, and available almost everywhere. I use it in a lot of the state parks, which are located quite a distance from the nearest town. It was not so a couple years ago, but they have added most of the rural areas into the system. I even got a fast connection while staying near Ainsworth Hot Springs in BC, where I assumed there would be no service. I keep waiting for the RV park service to become reliable so I will no longer need it, but there are just too many parks that have no service, or very poor service. Perhaps one day the park owners will get together and find someone who can provide a reliable national system.
I have used the Verizon system for wireless broadband since it was introduced. It is fairly reliable. But there are wide areas of the Western USA where you cannot get it to work.
I use Autonet Mobile. Great coverage and better than Verizon. It is 3G which may not suit some people but for our use it works fine. After taking 3 months for Verizon to correct a $500+ dollar billing error on their part, I wouldn't use them if they were the only game in town. I have yet to find a place in the east or west where I could not get internet access. I do have an external antenna mounted on the roof.
