I don't like to be "sold" on the higher level membership, but I succumbed at Costco and am glad I did. With the Executive membership, which cost $100 a year, you get back a percentage on most purchases. Things like US Postage stamps are excluded.
That said, I spend so much money at Costco, each year I get back over $400 which I then use to have a free shopping day there. In essence, the membership pays for itself.
They also offer a fee-free American Express card that rewards you with 2 percent back, even on gasoline purchases, restaurants, etc. I don't have that, but I have friends who do and swear by it.
My only beef with Costco is if you see something you like, grab it then, as the next week it can be gone and never return. That was the case with the Aero Chair, among other items.
Also, while their prices in general are lower than almost anywhere else, if you're a careful shopper you can find the same item cheaper elsewhere. An example of that is salt used in residential water softeners. At Costco a 40 lbs bag is $5.39, while at Lowe's it is $4.99. Not a huge amount, but something to take into consideration.