Don't have a large family (only DH,me, DD1, DD2, DS) I buy their wipes both floor and bottom wipes which although are more expensive £15 for 708 wipes they are bigger so I find I use only 1 at a time. I also buy their large boxes of soap powder. The last time I bought Fairy soap powder it was £20 for 90 washes.
I also like the quality of the meat.
I think if you do go to Costco then take a list of what you want and the cost/weight of what you are looking for so you can compare the prices.
We have a family of 6 and costco is more convenient rather than cheaper, I prefer to buy my loo roll every 3 months and know we won't run out, you can also buy somethings that the likes of sainsburys don't sell. However I don't believe I save money but the meat I agree is excellent quality.
We are a family of 6 too and we shop at Costco but only for the things that we buy branded, i.e. Marmite, washing powder, coke, toothpaste etc The majority of the supermarket value brands are cheaper, although the convenience of only shopping once a month (as it is bulk) can sometimes take over. The only thing we but that isn't branded is the big bags of flour, as we make our own bread (legacy of living in the US and not liking their sugar filled stuff) and the big bag is much cheaper. You can however get some good bargains every now and then, we just brought a new lawn mower which was about a third the price it should have been
We use makro but tend to go for their special offers. I stock up on washing powder, loo roll and I buy their frozen cheese cake etc. Lots of stuff is more expensive than Asda. They do sell bigger packs which is useful sometimes. Unfortunately we don't live near enough to a Costco to make it worth while. DH used to buy stuff from their when he worked close by.
Bought a massive gazzebo the last time we went. That probably used up any savings we will ever make shopping there.