I love Costco. Like other have said, love the gas savings alone. With 4 kids, we buy most of our sundries there - meat, school lunch stuff, pop, paper products, etc.
The only thing is, no matter what I go there for, I almost always end up buying something I had no intention to buy. It's like my impulse buying gene activates as soon as I walk in there.
Last time I went to Costco, it was an impulse buy of cheap 5-pack of socks for $6. Time before that it was a 2.5lb bag of M&Ms.