It is worth is just to use their petrol station.
This. We go once a month to fill up and pop into the store at the same time which usually ends up in a spend of £200 on stuff we didn't realise we needed.
There's a reason it often referred to as the '£100 shop'.
We routinely get our cat litter, loo roll, kitchen roll, dishwasher tablets, washing liquid, fabric conditioner, moist wipes, from there. I wait for offers on other stuff, then buy in bulk, e.g. Oral-B Pro-Expert toothpaste, Sure Maximum Protection deodorant.
Luxuries include cookies, shortbread, ale/cider. Meet when it's on offer - it's more expensive, but higher quality than most other places.
It's a lot easier to get the kids to go now they do samples in-store.
I bought my TV from there too. It was on offer and with their legendary service and 5 year warranty it was a very good package.
They suckered me into their premium membership a couple of years ago, so I get 2% cashback. So far, I've made the difference back in cashback each year, but as soon as I don't I'll drop back to the basic.
I'd say it was worth it on the whole, but as others have said do be careful as their prices aren't always great. Once I bought a wireless BT speaker on impulse, as a quick google at the time suggested it was a good discount, but after getting home and trying it out it was rubbish - I took it back and got a full refund, no quibbles. I did a similar thing with a back massager - it was good, but more research when I got home revealed it was overpriced - so I took it back.
Oh and I pay with my credit card. Non-Costco Amex. But I'm sure they accept Visa and MasterCard too. :S