The savings are good, sometimes they are really good. The cards work well at TA and Petro stops. There is a lot more to the NASTC program than just the fuel cards, but most of it applies to small carriers witht their own authority. Their fuel prices used to be on display to the public, but you have to be a member now to login to that website. I would ask for a trial subscription to the their pricing website or something so that you can see the price comparisons for yourself. I think they would allow it. Give them a call, I think you will find them to be very helpful and friendly... I did.
I have a NASTC account and fuel cards. I would do it again. They also do a good job of supporting the industry in ways similiar to OOIDA (not nearly as strong, but they are growing).