Only select items.
First - I agree, beware the Goldfish Factor - any of those items may very well lead to greater consumption, which will impact your waistline worse than your bottom line. And, in this category, chips, and snacks in general, you'll typically get twice the size bag for the same price as supermarket. For a large family, this might work ok.
If one is interested in saving on grocery items, the very first step is to get familiar with the unit cost (often cents per ounce) of most items you buy. Warehouse store or not, this knowledge will make you a better buyer.
In general, the papergoods/toiletries are cheaper than at the store but not as cheap as the big sale/coupon cost at the supermarket or pharmacy (CVS/RiteAid). So if you pay attention you may always be stocked up from other sources.
All that said, there are many items that easily cover our membership cost (for Costco). The meat, beef tenderloin, $8.99, I can pay up to $18 at the supermarket or butcher. Big shrimp (12 to the lb), $9.50/lb, easily $15 at fish dept. Funny, I buy the carrots JCarter mentioned. They are less than half supermarket price per lb, so I am ahead if we throw out the last 1/4 of the bag. More often than not, it's used up 100%.
Truth is, everyone will have a different experience at these stores. Costco will refund membership up to the very end, so why not try it, and see if the visit is worth it?
Last year, I read and wrote a review of a book titled The Paradox of Choice. The book's premise was the diminishing return that come with too many things to choose from. In my review, I observed how a benefit of Costco is the lack of choice, there's one or two brands for most items, not dozens. If you give this a bit of thought, it's actually a benefit.