Similar to what MCCOHENS said but the same goes for Costco.
Generally, .20 -> .25 cheaper than the surrounding stations, and if you have the Costco Visa, 4% discount on gas purchases (up to $7000).
The one problem with the Membership Stores is that they aren't exactly convenient when you need to fuel up while on the road.
I remember a Shell deal while on the way to Fl. in Oct.
There are other cards that probably offer similar deals.