The quality of the meat at Costco always seems to be superior to that of the local supermarkets like Safeway or Lucky's. Plus it's one of the few places where one can purchase pre-wrapped prime grade beef cuts. It's expensive, but it's nice to know it's available.
Even for a single person household these places can be good bargains. I mean, it doesn't hurt to stock up on nonperishable items like TP, napkins, paper towels. I also get my printer paper there. The clothing is fairly good quality, as well, although one must act quickly to get some items because Costco treats them as seasonal items - so if you'll looking for a warm sweater in the spring, you're out of luck. And of course the selection is, at best, a bit eclectic.
I also like their bananas and some of the other produce. But I try to avoid getting bulk food items that I know I'll probably wind up throwing out most of the contents. It helps to have a big chest freezer ;-).