I'm a Costco member through my mom's business and saved hundreds of dollars over the years buying my tires and filling up my gas tank there. Of course not every Costco has a gas bar, but I think most of them have the tire center.
The service is overall pretty good and I can usually get my car out within a couple hours (shopping the warehouse takes at least that long anyway!). When you buy your tires here you get the balance and rotation done for free and you can also purchase the (optional) road hazard warranty.
Friends of mine found such a great price on tires for their (his) car that even adding the price of the membership was worth it.