Costco is fantastic.
I buy all my autos from them. Fixed discounted pricing, no hassle purchasing.
For example, I was in the market for a new truck. Stopped by Ford Dealer to test drive one. We negotiated pricing, but I wanted to do a bit more research before buying. When I went back the next morning, they said all these different items that were "included" the day before, were now going to cost and additional $5K!! I walked.
Contacted Costco Auto, they sent me to a different Ford Dealer. 2 hours later I was driving away in a new F-150. While finalizing the paper work, the sales guy says "let me see if I can find more discounts for you". He searched through the computer and asked if I had brought in the coupon. Of course I said yes, and he said "great, that's another $1000 off". It's night and day difference buying through costco's auto program.
That aside, even with groceries, it doesn't take too many trips to justify the cost of the membership.