Their personal guarantee on electrical items is amazing. Our tv button broke, rang Costco, they sorted out someone coming out, after job is done, Costco ring to make sure you are happy with the service you received. Tv was out of manufacturer's warranty, but not Costco's.
Costco is on the way home from work for Dh so we have shopped there for years. The way to do it is research supermarket costs and take out the Costco American Express cashback credit card which we only use in Costco. Plus we also get cashback by being a premium member.
Washing powder works out at 10p a wash compared to 17p in supermarket.
Milk, a 4 pint is sold as a 2 pack for around £1.70, so cheaper than 99p for 4 pint deals in supermarket.
Toilet roll/kitchen roll/tissues - cheaper
Wraps and bread - cheaper on actual brand comparison
Smoked cheese - cheaper and again brand comparision
Philadelphia is ridiculously cheaper
babybel cheese
normal cheddar
Any big packs of fresh stuff you have to learn how to deal with, so 6 pack peppers (red and yellow) cheaper than supermarket, but you chop it up and roast it with red onions, courgettes and tomatoes and freeze it in portions. Lovely with salmon -also from Costco.
All their meat, fish, chicken is just better quality. We do have a larder freezer though so we portion it and freeze it. And I have a pantry for tinned tomatoes etc that come in large packs. But we knew we would need room for Costco stuff before we bought this house 