I'm a member. I've never had my own membership, though. I've always shared one with a friend or roommate (you get two cards, one for you, one for your "spouse" for free).
Costco gas prices in SoCal are at least $0.15 cheaper per gallon. I buy lots of food there too. The queen-sized Sealy Posturpedic mattress I bought there last week for $499 is $860 at the Sit N' Sleep across the street.
I wish they were open 24 hours a day so I didn't have to compete with the people and all their kids.