We have Costco, Sam's and BJ's in our area. Some goods, like detergent, paper products or bottled water, are well priced for the amount vs. what I can get at local stores or Walmart. On the one hand, the proces pretty much stay low, though they always have coupons and specials running. The bad thing is you buy in bulk, so while you can get a great deal on, say cereal, you have a HUGE box of cereal you have to go through or have it go bad.

Brand selection is also a factor. Each store has their own house brand, which may be fine. But if you are particuular about your katchup, toilet paper, aluminium foil etc, it pays to have a look at each store and see what they have and what you can afford to switch out.

I do a bit of grilling and smoking year round, and their butcher section is usually better than I can find at local markets. Even then, I have to figure out what store carries what. For instance, I can get racks of ribs anywhere, but Sam's and BJ's have 2 packs, and Costco has 3 packs, often for the same price. If i'm cooking a brisket, Costco has a better selection of whole packers vs. flats (flats can be found at any store, but I've a better chance of getting the packer (flat + point) at Costco), and even then, it depends on which Costco I go to if I want, say, a Prime brisket over a Choice brisket. Beef short ribs are good, but I've only found them at BJ's, and even then, only 1 of the 3 in my area has them in the case.

In short, it can be worth it, but you have to know what you want and which store carries it. Have a look around before plunking the annual membership.