Giant Eagle ("Jine Iggle" in local parlance) is the big supermarket chain here, and I've cashed in on the program once so far. I ended up rolling into the station on fumes so I could maximize the reward (they only discount a single fillup) and ended up getting a 14 gallon tank filled up for $0.59 a gallon or something when gas was $2.30/gallon or so .

It's a decent reward, but I can't imagine it's any better than some of the rewards/discounts other supermarkets give with their loyalty cards. Plus, since it's only valid at Giant Eagle's own stations, they could presumably jack up the base price of the gas to offset any losses from the rewards program (not that I've seen evidence of that.)

But yeah, it's cool. I just don't do enough grocery shopping to cash in very often.