we go regularly. when the booklet of special offers comes through, i go through it and buy anything on offer than we actually use, even if sometimes we don't actually need it right then eg. loo rolls, oxywhite powder, kleenex tissues. i buy all my meat from there as i think it's far better quality than any supermarket, trays of baked beans and tinned tomatoes, huge rolls of bin liners, catering packs of chicken thighs, roast ham, roast beef, whole sides of salmon, scones (all of which i split and freeze in portions).
you do have to be a bit careful with some things though i would agree. due to personal taste i prefer asda's own brand tuna, so never buy tins of john west from costco. when we first joined, a long time ago, i used to go mad and buy loads of stuff, some of which would get thrown out eventually. these days i'm a lot more selective and it really does save me money.
if you're a trade member you can claim the VAT back which makes it even cheaper, but even as an ordinary member i reckon you save money.....providing you're choosy about what you buy. the criteria for being a member is pretty wide, i'd advise anyone thinking of it to go along and make enquiries.