Sorta got talked into it by new neighbors when moving here in late '14. Joined in Jan 2015 and right away bought an 80 inch TV for $3K.
Got the exec membership for $110 and combined it with their Amex which yielded over $850 in cash back to date.
But excepting the cash back from the TV buy, most all the cash back was due to purchases not done at Costco.
There are only two of us and little to nothing available at Costco makes sense for use to buy it. One cannot comparison shop, one needs to buy large quantities, and we don't like the crowds. We pay lower prices for most everything elsewhere, including gas. Since in Colorado, the Costco has no beer or wine or booze.
I thought their tires might be a buy but for the exact same item Discount tire was $140 less for a set of four.
So we are quitting. We got tempted into impulse buys a few times, and regretted it. No more. One less piece of plastic in the wallet.