Pros.
It is very well run, they sell high quality products and have favorable return policies. If you buy high ticket items like a Dyson vacuum cleaner that breaks they will fix it or trade it for you without a receipt because they can look up your member number.
Going to Costco feels like “an event” for my kids, we get pretty decent lunch for a great price and it’s always fun.
Anything that is non perishable or that you know you will use is a great deal. Things like paper towels, detergent, milk, eggs and roast chickens which is my go to healthy meal.

Cons.
I think people purchase things foolishly at Costco because they think it’s a good deal. Don’t buy anything that is perishable unless you know you will consume it. My mom always buys a case of peaches or avocados because, “it’s such an amazing deal,” and eats one or two and the other waste. Sometimes you just want two oranges, and you get sick of them before you eat the other 22. My wife will be craving “a little hummus,” and you eat a few bites and it pains me to throw away this giant thing. Also, never buy something at Costco that you are just trying for the first time because it looks good. Ive made the mistake of buying a 24 pack of some healthy chips and they are terrible and now you’ve bought 23 that you can’t get rid off. It’s like those “fun packs” of chips; it was never a good deal because everyone eats the Doritos and Cheetos right away, and then you are stuck with plain lays and ruffles that no one wants.
So buy smart and it is a good place.