I shop there quite frequently. Although there have been many items that are cost efficient, unless you have a warehouse in your house, I think it is not as cost efficient as most would like to believe.
If you are having a party or are the Brady Bunch, sure, buying in bulk is awesome. But many times Costco does not carry the same items all the time. I have found that it is a "crap shoot". If there is a certain item that you like to buy ther, you might want to buy alot because it may not be available the next time you shop.
Also, the lines...UGH! There are never short checkout lines and yet there are tons of empty registers, why is that???
I guess it is all a matter of opinion, but I do not feel the $50 per year membership fee is worth it.