Welcome to the superstore, where you can buy a one years supply of anything you may need or want. It may not be the brand you want, it may not be size you want, but it will be there.
I had never had a Costco membership, but did not necessarily envy those who did. My wife and I try to limit our purchases of perishables as we do not cook that often and when we do, we are only cooking for two. The thought of purchasing 12 packs of soup or 18 packs of Izze seemed a little outlandish, but I had to admit that we would be saving $5 on the soup and $5 on the Izze by buying bulk.
The electronics left a little to be desired. Again the selection of the merchandise was an issue, but the discounts itself were not awe inspiring. For example a Vizio 42” LCD television could be had for $1099 which seemed like a good price and was indeed a $100 discount relative to the Vizio website and you do not pay for shipping. However Wal-Mart offers a Vizio model for $898 which is an $80 discount relative to the Vizio website.
Unfortunately I did not venture into clothing or home accessories but I believe the net would be the same. There is a substantial savings relative to usual outlets, however it is no more than one would get by keeping an eye on the circulars. The $20 membership is reasonable, even if the shopping experience leaves you feeling like cattle.
Also be prepared with American Express, cash or a check as no other major credit cards are honored. This is a mild inconvenience but it affords Costco a 2% margin improvement.
The largest benefit is for small business owners and those with large families who prefer to buy in bulk and would rather shop once for the month stocking up on non-perishable essentials which can be stored in cupboards and basement shelving. It is much easier to grab a 12-pack of one quart Gatorades from Costco than it would be to accomplish the same task at King Soopers.
Holiday entertaining is the other scenario where Costco offers major benefits. When cooking for 20 people don’t try to get appetizers from your local grocery store. Unpacking 4 servings at a time will drive you bonkers. It is much more convenient to have bulk serve packaging. As far as the half gallon mayonnaise for home use, that is another story.