I have a free membership at Costco through my work, which I use a couple of times a year. I did get our cell phones there, and occasionally meats, seafood and specialty grocery items (usually stuff I don't need that just looks enticing.) And they sometimes have exceptional wines for a good price. I hate their milk packaging - I know they are a space saving design but my kids end up spilling it all over. Also, my daughter's a competitive swimmer and they usually have Speedo suits for less than $20 each which is a great deal. I don't like that they don't take credit cards and I don't need an Amex. I pay my bills off every month, but there's something about me that just likes having that float time that credit cards offer (as well as the rewards I get back.)
I still pay for a Sam's Club membership on top of it; I've actually had it for almost 20 years. I shop there several times a month, usually for staple items. I can't count the number of cakes we've bought there (while not over the top spectacular, they are very good, and an extremely good value.) Over the years I have bought several large electronic items, mattresses, and furniture there. And oddly enough, many of the employees there have been there as long as I have been a member. So I can't imagine they treat their employees badly.
Oh, and both stores are within a few miles of my house, so no advantage either way there. I agree with others that Costco is more upscale. But I end up giving a lot more business to Sam's Club.