I have a Costco membership and I love it. I'm single so I can't buy the huge vats of things but here's my list:
Litter
TP
Paper Towels
Chicken Broth
English Muffins
Mushrooms
Books
Photo processing
Tires
Flowers

On top of all the savings, I have my little sister on my membership for free so she gets to use it as well.

As far as paying for a membership/club to something like Barnes & Nobel, I think that's ridiculous! You could never save as much as you do via Amazon or Borders that doesn't charge for membership.