I only belong to Costco, and I absolutely love it. Like the poster above me, you have to know prices, and know what is cheaper. Cheaper at Costco than the grocery store for me are: butter, bacon, most cheese, maple syrup, fruit, half & haf, organic milk, eggs, dishwashing liquid, laundry detergent, any baking supplies, most spices, and I believe thats most. I know that sounds so unbelievably unhealthy, but I don't eat butter, bacon & cheese everday!
The customer service, and their own brand Kirkland, cannot be beat. Excellent quality, and excellent customer service. I am never giving up my Costco membership!