I have a membership at CostCo & love it!
You do have to be very careful, some things work outmore expensive, & always read their labels carefully, some non food items say ex or inc of VAT.
I buy all toiletries & washing/cleaning products/toilet roll/baby wipes etc when they are on special offer in CostCo. I cant afford to bulk buy, but unlike Bookers for example, the "bulks" are alot smaller. For example, a 3 pack of Dove deodrant, a twin pack of Fairy non bio. Bookers tends to be packed in much larger amounts.
I dont purchase much in the way of fresh as i tend to shop for fresh things as & when, as im a single mum with only 2 of us in the house. Although they do have a brilliant butchers section & sometimes i buy meat & freeze it. I spmetimes get onions & potatos as they dont go off too quick.
I absolutely LOVE their bakery & ready made section. I always treat myself, i love their Take & Bake pizzas!!
I also think that when i need a new vacuum or other household item, it will probably come from CostCo as their TVs/kitchenware etc seem cheaper than other stores.
When i first got my membership (in a brand new store that opened near me) i got a 30 day free trial. Are they offering anything similar at your store?