Funny to see this today, as I was having a chat with some Costco folks yesterday at a farm show in Scotland. We don’t really have a Costco close enough to make it worth our while, but my wife used to visit one from time to time on a relative’s card.
What we found was that the cheaper prices were more than outweighed by the temptation into additional purchases that we would not otherwise have made - packs of flavoured water in 250ml bottles, snacks, confectionary etc. So probably not a good option for us.
Other folks have suggested that using Costco for fuel and tyres is well worth it, if the store is near you and you are not looking for specialist tyre choices.
I gather their on-line store is quite good, if you are in a location with good delivery services, and I think you can try that out without taking out a full membership.