Well, I pay something like $45/year for my card but I'm convinced that it saves me money. I'm talking about Costco here.
I just shop for myself. Walmart? Kind of a goofy place in my experience and they are a lot longer drive for me, anyway. Costco has a lot of stuff Walmart doesn't have.
There are a lot of things I get at Costco that would cost me a lot more every other place I know of. I've checked out Amazon a bunch of times, mostly the deals aren't good for me. They do have some things that Costco doesn't, it happens, but I rarely buy food off Amazon, it just doesn't pay, is my experience, and food makes up the biggest part of my Costco expenditures. There's a ton of stuff at Costco (most all of it) that I don't give a 2nd look, but I've gotten a lot of very good things there (food and otherwise).
Costco's kind of an unpredictable thing. There are items you can count on them having, but you can't count on everything. There are some things, a lot of things, that are there today, but in a month or two they won't be. They are more that way than most chain stores.
If you have their item number for something (or even if you don't), you can call them and ask if they have the item, when it will be restocked, and if other Costco warehouses presently have the item. I've done that many times.
Costco's pretty good for gasoline, that's where I get mine almost always (I don't drive much, so most times I hit Costco, I just fill up before going in the store).
There are many food items that are just a better deal at Costco than anywhere else I know of. I used to get nonfat dry milk there, but Walmart is absolutely cheaper for that and they'll ship free if you order $50 worth of qualifying items. I get my total over $50 whenever I get more NFDM, therefore, shipped free. I have had more than one bad experience with Walmart shipping, though. Apparently, the guys in the warehouse are kind of goons. The NFDM boxes can be broken, they shipped me the wrong item of crackers last go-round. I called, and they reshipped me.
There's a different vibe in Costco than in non-member chain stores. It's a little more friendly, less alienated. They have all those food sample things going on, I like those. The people you see, they are members so you have something more in common with them than if you were in a Walmart or Target. I think that's a good thing. There was a thread here where some people bitched about their policy of glancing at your cart and putting a slash mark on your receipt as you exit the store, but it doesn't bother me, really.
Oh, and another thing: Costco's return policy is pretty fabulous. I haven't used it a lot, just a few times, but if you buy something there (well, most things) and aren't satisfied, you just bring it back for a full refund. I don't think you even need the receipt. They can look it up based on your membership card and their records of your shopping.