My partner and I have Costco cards. I do like Costco and a lot of the things are quite a bit cheaper than in normal supermarkets (though I find that fruit and veg are similar prices). I have found the cards to be handy as I can take my mum shopping there and she can stock up on loads of meat, household goods (decent quality toilet paper is really cheap and washing powder / fabric softener is cheap too), alcohol, soft drinks etc. Like AnnaFaraday said, if you've got the storage space for it it's great but if you're only going for a few things it's probably not worth the membership.
If you've got a friend or family member with a Costco card it might be worth asking them to take you one day (you can take 2 guests in on one card) so you can see it for yourself and then decide on whether you think it would be worthwhile for you.