I just like them. Their meat and seafood is always high quality. Their Kirkland products are great. You see the same staff there year after year. As a single person it wouldn't normally make sense for me to buy in that kind of bulk but I get stuff for my parents so I think it save enough to justify the basic membership I have.
What pays for my membership is buying gas - I save at least $0.20 / gallon on the low end for regular, and even more on premium; it's about a block away from work, and the gas station opens early, so I tank up during the week, and watch everyone else wait in line starting Friday afternoon all the way through Sunday afternoon. Between my Costco cash refund and my Costco AmEx annual rewards check, my membership fee is covered and I normally have enough for another shopping trip. This year it paid for the new netbook.