I haven't read this whole thread, just the first 2 pages and the last page, but I would be very cautious about using this service for several reasons:
1) Sprint has roaming agreements that cover just about everywhere with their agreements with both AllTel and Verizon. These agreements I believe are backwards compatible, so if you have Alltel or Verizon, you have access to data through all 3 networks as well as any smaller networks they have roaming agreements with. Also note that roaming is no longer something to pay for with data.
2) The initial map shown by this company is a Verizon map, so they aren't doing anything extraordinary. They are just piggybacking an existing cell company.
3) Its important to have the latest PRLs on your device to make sure you're going to get access to the towers. If you have data with a company directly, and are up on updating your device manually (which is easy to do and updates it more frequently than the 'push' system they usually give you for software updates) you are more likely to have better coverage than relying on something that sits in your cig lighter that may or may not be something you can (or easily) update with the proper PRLs.
My advice is get Sprint, Alltel or Verizon and then you know that you're with a company that you can have more control over with your device, and don't have to worry about the contract or terms that this company may have to use 3rd party data network.
I'm Sprint's number 1 fan. I used it for 10 days this past spring across I think 9 states and honestly can't remember losing data, even in the most remote areas of texas, oklahoma, western kansas which used to be vast nothingness.
I'll add one more thought... the technology is rapidly changing with wimax and EVDO rev B coming in the future... I would just stay on top of your favorite phone company, see who has the most and best roaming agreements in place and just upgrade your device for the fastest/backwards compatible equipment they offer each year.
Andy