If you aren't a heavy gas user, the Discover - Open Road Gas Card can save you an extra 5-10 cents a gallon on gas. You get a 5% rebate on gas and card repair purchases, but this only applies to the first $100 spent each month, so its not as great if you spend a lot each month.
The nice thing about Discover Card rebates is that you can get them as cash as well as getting gift cards, etc. For some things like movie rentals, you can get up to double the value of your rebate.
I like having a separate gas card because when you use it at the pump, the station can put a hold of $100 or more on your account and this can cause problems if you use a debit card.
Discover - Open Road Gas Card is recommended by dadown