Costco opened a store a few years ago up here in Newcastle/Metro Centre, and basicaly was no cheaper than anyone else and what was cheap was rubbish. Closed down with in the first year.
We have one down south, it's like any supermarket these days, if you happen to stumble across a bargain (within your normal shopping) then you're sorted. However that kind of thing is too random to make it part of a reason for choosing one store over another.
Costco next to the Metro Centre is great for stocking up on meat, brilliant quality.
Don't know where oldphart got the idea it was closed from?