Meh, ever since I discovered Amazon Prime, I no longer care about Costco.
I do both - Amazon is great for those lazy days in particular.
A married couple friend of ours just joined. Apparently he drinks something that never goes on sale or with coupons and the savings alone on that was more than the cost of a membership.
During the summer the produce prices are about dead on with the other places. I prefer their produce or Whole Foods because both seems to be of better quality/last longer. Of course that is totally regional.
Milk is always cheaper there (or on par with most sale prices) but I hate the milk jugs.
They have kick ass lemonade in the summer that I like that is ridiculously cheap for organic lemonade.
I prefer to get boneless chicken breasts to cook with (not a bone person) and again their chicken is cheaper than what I can find on sale (at least ones that are either organic or don't feed extra hormones to the chickens). Other meats I don't bother with as I can taste the difference in grain fed local meat vs not. If they step up and match what I buy elsewhere in 'quality' I'll buy.
My Dyson and KA Mixer came from there - all at a substantial savings.
I eat a specific granola bar for breakfast every morning and theirs again is cheaper than store with sale or coupon
I bake a LOT and they are now carrying a flour that I really am liking (ultra grain unbleached all purpose)- comes in 2 10 pound bags. I buy the 10 pound bags when I go to the grocery store though so that was normal to me in size. I had been buying an ultra grain at Whole Foods in the bulk that was 79cents a pound. Costco's is 37 cents (IIRC). I do go through that quickly.
Yeast - OMFG it's like $2 for a huge bag. I just freeze it. Again I bake a lot and use yeast at least 1-2x a week.
They carry name brand clothing for some items. We have bought our son's Levi's there since he was 2 or 3. They also carry Carter's for the really little ones. Great prices on winter coats. They carry name brands for men's clothing in addition to their Kirkland brand. I admit I don't buy there because they don't carry a wide range of sizes. I bought 2 down coats there - didn't care that they were a bit large on me. For an average sized person they carry good deals.
Socks are also a good deal there for us.
One thing I don't make on my own is ravioli and they have a frozen ravioli that isn't too badly priced and tastes great. Same with frozen meatballs (fantastic for parties)
Not bad deal on shoes if they carry the brands/types you are looking for.
My brother has purchased furniture there and has been thrilled with it.
Hygiene products are pretty cheap as are medicines, contact solution, vitamins etc. Biggest issue is not a huge selection but if you use the brand they sell it's cheap.
I actually find the Kirkland brand of facial wipes and face serums to be great and inexpensive. Usually paraben free and not tested on animals.
That's my rattling off of what I like to buy at Costco. When our son was in diapers and had to be fed soy formula they really helped.
Oh I buy toys there - lots of toys. Right now we have a 4' tall huge stuffed dog that I got for some ridiculously low price (like $30).