Where we are, we have an 'executive membership' with Costco. It is more expensive, but we get 2 - 3% back on everything apart from alcohol, petrol, and cigarettes. The refund comes as a gift card once a year. I use it to pay the next year's membership and the refund is usually high enough that it pays 4/5 of the membership. I don't know if that is an option where you are though.
The Costco business model is they only stock around 700 items and make a 15% profit on everything. This means that no, they won't have everything you would find in a supermarket. I find that they are normally a lot cheaper than a supermarket, particularly for things like alcohol. DD also likes the 'Madras Lentils' and we get those on sale at Costco at roughly a third what they cost in the shops. They do stock consignments of things (such as Orvis jackets and toffee pretzels) that you'll see for a month and never again which can be frustrating. Their own brand (Kirkland) is of excellent quality - I think the vodka is Grey Goose. It is worth keeping an eye on supermarket sales though as when they are running loss leaders, they will undercut Costco.
We usually do one big shop a month when we happen to be on that side of town so it isn't too inconvenient. We buy household staples like toilet roll, paper towel, bread, coffee, cleaning supplies, oats, cereal, pet food, flour, sugar, some spices, bottles of water for lunches, snacks, toiletries and laundry supplies. Even without meat we still spend about two hundred a month in there and probably another hundred and fifty at the petrol station.
I am very much in favor of Costco. They are an excellent employer too.