Is it really worth joining a warehouse club like Costco?

Don't expect to save money on everything - there's stuff where Costco costs the same or even more than your local supermarket. It's really only worth it if you're buying in bulk regularly (large family, stocking up on food), otherwise you'll never make back the annual membership fee. Before you sign up, look at what you usually buy and compare prices - it'll take you 10 minutes and you'll know if it makes sense for you.