Costco is only worth it in certain scenarios. But I see tons of people going there and buying things that actually aren't a savings at all, but they've convinced themselves of it just the same. Personally I haven't enjoyed the experience of going there, and the occasional item that has turned out to be a deal hasn't come anywhere near making up for the cost of membership, so I don't think I will bother renewing it anymore. For those people who buy quantities of specific items that actually are cheaper at Costco, it's worth it.
Back home in Quebec, they used to carry cheese curds. They weren't really any better of a deal than what Walmart offered, but I liked them. Here in Alberta, I haven't been able to find cheese curds in grocery stores or at Costo, a damn shame!
All in all, I think it can be worthwhile for some people, but not most. Every time someone has pointed out a specific item they felt was a deal at Costco, they were either being tricked by the size/quantity and don't know how to do math, or they just were too lazy or failed to properly search for similar items elsewhere. If you buy sufficient quantities of the few things that truly are a deal there though, go nuts!