What can be said about Costco that hasn't been said already? Bulk items. Cheaper since you're buying such a large quantity. Good customer service. If you haven't tried the sundaes at the food stand, they are amazing and they're under $2. I get one every time I go.
One word of advice: try and avoid Sunday afternoons. Its filled with yuppies and their pack of kids and friends and relatives. Way too many people for me, but if you like the crowds, go enjoy the people watching!