My husband and I belong to it - no kids. My dad bought us our first membership, and i wondered if it would really pay for just 2 of us...
Some things are great - the quality of meats are excellent. I find Costcos prices to be cheaper than our grocery store's regular prices, BUT if our local store (Publix) has a sale, it tends to be better than Costco. We buy frozen fish at costco too. i was never a person to cook fish before, but the vacuum sealed frozen packs really are great!
We do use costco for things like laundry detergent, cleaning supplies, aluminum foil etc. The pricing works out better, though of cuorse with 2 of us it takes a while to go thru the quantity from Costco.
We also like the other random items that rotate - i have 2 of the comfiest kitchen mats ever now, and they were like $10 each. We saw similar ones in bed, bath, beyond for like $50.
And of course, if we throw a party, costco is great with veggie platters, shrimp platters etc. So much better than at publix!
Can't comment on the diapers and formula since hubby and I have only been married a few months

But for the other things, it does seem to be worth it, even just for 2 of us.