I have been a member since it opened. The way I explain it to people is that you get twice as much for the same price. So you need a large family or a shed (I have the latter) and a freezer. The lack of some products is frustrating (eg brown sugar, pork fillet) and the deli area can be a little volatile depending on what their buyers find - I bought a fantastic salsa one week and have never seen it again.
The best savings to cover your membership?
First, any Kirkland stuff (their homebrand) - always well priced and excellent quality.
Second, my biggest savings are on dishwasher tablets, laundry powder, cat litter and cat food (the Signature cat kibble is as good as Iams and a quarter of the price) and 36 month aged parmesan (or padana if I'm broke). I am led to believe by Mrs_G that feminine hygiene products are also keenly priced. Favourite other items are Echire butter, french and spanish cheese, the hot dogs and ALligator gnocchi (the brand, not made from)
Oddly, fruit and veg is really expensive. Electronics are OK but you can usually do better if you haggle at a Good Guys - but if you are the kind of person who pays the sticker price, its a good buy.