I used it years ago but not currently.
The negatives are that the discount does not apply at your home base airport, and it generally can't be used for self-serve. The discounts were on the order of 25 cents per gallon, maybe 50 cents rarely, so often self-serve was cheaper than full-serve with the discount.
You also have to remember to ask for the discount, and the FBO person needs to know about it and remember to give it to you. Neither of these happens every time.
The application would be if there is an FBO you visit frequently, or if you fly a lot of cross-country.
The cost is $39 per year. Their terms allow for a refund within six months of purchase, so you could try it and not lose much.
Jon