I love Costco! Even as a single guy there is plenty of things I can stock-up on and save money off my grocery bill. I come here to buy things you know you'll go through so paper products, contact solution, tooth paste, etc...
Sometimes they even have decent brand named clothes for cheap. Need a pair of jeans to do work in? They have them for under $15.
My membership pays for itself with the free tire rotation you get when you purchase tires from the Costco Tire Center. You first save money buying tires here, second you save on not paying for rotations and any flat repairs. Totally makes the sting of the $45 membership fee disappear.
You also gotta love the food court, $5 can buy you a stomach ache with $1.50 hot dog combos and giant slices of pizza!
The only disadvantages is how crowded this place gets on the weekends and how they change products frequently, so if you like a certain product don't bank on it always being offered.