Where Costco really shines for me is dairy. Sometimes I wonder if we shouldn't buy a cow because of how much we spend on dairy, but for right now, I can't get cheaper prices anywhere else on cheeses, cream, Greek yogurt, and milk. That alone pays for the fee.
I haven't found the meats to really be any cheaper or that much cheaper than any supermarket's sale. I love the Kirkland frozen chicken breasts and buy 2-4 pounds of fresh cod or rockfish a month. If it is a really good deal, maybe some pork loin or shoulder. The pickled herring is good and really cheap.
I buy 95% of my beer there and will probably buy most of my liquor there now that they sell it.
I generally don't buy any of the convenience frozen foods. Too expensive for most items. I stick to frozen fruits and veggies, bacon, chicken breasts and do my best to ignore the rest.
The produce is generally good. I usually buy peppers to freeze, mushrooms for the crockpot, apple slices, and onions.
I tend to avoid the non-grocery sections of the store unless there is an excellent deal on something I actually do need. Doesn't happen more than 2 or 3 times a year though. In the past two years, I have gotten a good rice cooker, memory foam mattress (online), and two food storage containers sets. I try to buy Mobil 1 whenever it goes on sale.
Gas is not usually any cheaper at Costco than I can get it wherever else and I don't have to wait in a ridiculously long line for it. Most supermarkets with gas stations have cheap gas plus loyalty discounts.
It is worth it to my family, no doubt about it. We rarely waste anything from Costco.