I shop at Costco every week. I rarely enter any other store. Off the top of my head...
1. They do not sell gasoline at cost. However, it is usually a few cents below other gas stations in the area plus you get another 3% off (that's $0.09 on $3 gas) using the Costco AmEx. The upcomimg Visa card gets you 4%. Gas is Tier One i.e., higher additive levels. I've used a lot of their 93 premium. It's good stuff.
2. As others have said, I more than recoup the cost of my Exec membership in rebates. So, the cost of that is a non-issue.
3. I do buy my oil at Costco. Mobil1 is on sale every other month for about 4.50/qt (in six quart cases) which is lower than Walmart in jugs.
4. Best place in town for warm beer and wine. Costco is the largest wine retailer in the world.
5. Produce is typically very good. Sometimes quantities are too much for a couple.
6. Meat department is outstanding. Not cheap, but competitive. Small selection of prime meat is available but their choice steaks are so good, why bother? I like to buy whole NY strips and cut myself. Pork tenderloin, bacon and ribs are great. As is the chicken. Good farm raised salmon. [censored], it's all good.
7. Drinks. Every week I buy Diet Coke, bottled water and Mich Ultra. Life's staples.
8. Paper products, cleaning supplies really good when on sale.
9. Frozen and dry goods are great prices if you can deal with the bulk. $12 for a large frozen lasagne! You can't make it yourself for that. Items like mayo you can save $4-5 on a single large container. Spices, BBque sauce, hot sauce, canned tuna, soups, anything in a can.
10. OTC drugs, vitamins etc very competitive.
Other once in a while stuff like computer paper, printer ink, yard fertilizer, batteries, wiper blades, TurboTax, softener salt. I mean the list goes on and on. Why go anywhere else?