I signed up as a member months before the Sydney store even opened. It did help that the sign up office was in the same centre as my "local" Spotlight.
When Costco opened on July 21, I swore that I would avoid the shop for a few weeks until the crowds died down, but in the end I didn't even last a week. For the first month it was open, I think I went once a week. Now I've cut back to once a month. I tend time my trips for when I know it will be less busy like close to opening time in the morning or after school in the afternoons. I never go on Saturday or Sunday.
I get asked if I think it's good value. Obviously I find value in shopping there, but if you are good at shopping sales at Woolies and Coles, you can better deals with their sales sometimes. For example, Lurpak ran a promotion with Woolies where you get two blocks of Lurpak unsalted butter with a loaf tin for $5.99. At Costco a 3 pack of Lurpak is $10.29 ($3.43 each) which is a lot cheaper than the normal price at the shops which I think is about $4.70. If you buy a lot of homebrand/store brand merchandise, you might not see the same value.
Organic milk is a good price when they have it in stock. It's $3.99 for 2 litres instead of $5.12.
They also have American products like Hershey's syrup, Dawn dishwashing liquid, Dr. Pepper, Monterey Jack cheese, Hershey Kisses, Ghiradelli chocolate, dried sour cherries, etc. I really like the organic salsa. It tastes heaps better than any of the store bought salsas in Australia. It's also a great thing for me when watching my weight as it adds flavour without too many calories.
Kirkland is Costco's "homebrand". Usually their homebranded items are very good quality.
I also like buying bigger size things, so I don't have to buy as often. I don't mind having a 300m roll of Glad wrap or buying 48 rolls of toilet paper (stashing it around my house can be challenging). And the best "big" thing, a 500g bag of Cheezels!
There are clothes, books and homewares. I tend to always have a look at books when I go. The book prices are similar to Big W's book prices.
What don't I buy? In general I don't buy meat because none of it is organic, and lots of it is grain fed, so I choose not to eat it. They do have free range chicken, but I prefer organic chicken. They have free range eggs which I would use in a pinch, but yet again, I prefer organic. It is the same with fruit and veg, but if something is a good deal or I can't readily get it organically grown then I will buy it.