I've been a long time costco member. When I was a single guy sharing a house with two other guys, buying everything in bulk didn't make sense if I just wanted stuff for myself. I did convince my roomates to chip in towards my annual membership and we would pool for stuff like paper towels, toilet paper, and other "common use" items.
Now that it is me, my wife, and two dogs, Costco still seems to save me money in certain areas. I still know what not to buy perishable wise from costco, because I simply won't be able to eat it in time. For me, I cook 90-95% of the food I eat, with very few occasions of going out to eat or picking it up from somewhere (I like to know what is in my food, and I like to cook, and I eat a lot by virtue of my active job and outdoor hobbies). Things I buy on the regular that I haven't been able to beat elsewhere:
Eggs
Egg Whites
Jasmine rice
Oats
Chicken
Frozen veggies
Oils (avocado oil is my new favorite)
Nut butter
nuts
dog food
trashbags
laundry detergent
Avocados
Spinach
Wine
Their return policy is pretty unbeatable by any stretch of the imagination. I've been able to return things there no questions asked many times since I have been there. I also look at their deals for Travel (vacation packages, car rentals etc) to see if I can beat that or not based on where I want to go. Car rental discounts have been pretty noticeable for me, but not sure how many people know about it and/or need to rent a car on a regular basis. I am also fortunate to have the tire store attached to this Costco and those seem to be much cheaper than other standard locations. Lastly, I have noticed that certain Kirkland brand clothes (dress socks, underwear, undershirts etc) are much cheaper there and fairly good if not excellent quality. Lastly, the gas is always cheaper than any other gas station in the area, and it is on our way home from work. Do we go out of our way to get gas there if we need it? no, but planning ahead helps a bit.
Just like everything else, it can be mustachian if you avoid the pitfalls of shiny, unneccessary stuff. And so long as you go in with a list, you are less likely to impulse buy. Have I been guilty of buying some of the random items at times (like dips or frozen prepared meals) yes, but usually everything we buy gets used/eaten with little exception. And with the executive membership, I haven't paid for a membership in a few years, and usually they pay me. If we move to a location where an Aldi's exists or other viable means and Costco is out of the way, we might rethink our stance. But for now we have the means to buy in bulk and buy what we need in bulk, as opposed to what we might want.