You might want to check out Virgin Mobile (
http://www.virginmobileusa.com).
Virgin Mobile is the same as the Virgin Airlines company... odd combo I know, but at least it is a known, reliable company.
I believe they piggy back on the Sprint towers (I could be "misremembering" that but it is one of the other major carriers they piggy back on). In the US we've always had good coverage.
We have the cheapest option they offer - we add $15 every 3 months (it ends up being $15.75 w/tax). Per minute it is a bit high compared to other plans, but if you are like DH & I you'll never use it all. We both have over $100 balance on our phones right now.
Downside is you buy your phone...unlike getting it for free when you sign on for some of those 2+ year plans from the other companies. However, I still think over the years we have come out way ahead compared to paying the monthly service plan amount to Sprint, AT&T or Verizon.