My Costco rule: If our family eats it all, then it's a good deal.
I always buy the same stuff, pretty much. I like their Aidell's chicken sausage (great deal!), Babybel cheese, the pesto, bagels and muffins, wine, olives, toilet paper, bottled water.
Their dairy prices are on par with Wegman's, so I alternate.
I don't buy sandwich bread and lettuce at Costco because they usually go bad before we can eat it all. I get those things at the regular grocery store.