Depends on where you live. Where I live, (secret base) Costco is more expensive for groceries with a few odd exceptions like maple syrup and organic olive oil. Random stuff like that. Not worth a membership for me - I cancelled several years ago and have saved hundreds of dollars in membership fees. I have relatives who pick me up the occasional odd thing like the maple syrup.
The real unslick thing about "CostMo" as we call it, if you watch how almost everyone there shops - is the impulse buys. I "cart watch" and people watch at CostMo sometimes - it's an interesting place to visit. Also, many of their products come in higher quantities than you might normally buy. For the average human brain, this means you eat more of it. At my local grocery store, the smaller boxes of many common groceries are actually cheaper per ounce/pound/whatever than larger ones so you can't even assume the price is better on Costco's giant packs. And you lose out on sales if you buy in bulk.
But that maple syrup, man, it's like half price.
I totally agree with you except that maple syrup. Why do you feel the need to eat that much syrup when you have sugar there available and free (well almost!) LOL
On a serious side, I don't think it is cheaper when you buy so much of it (everything in Costco is a giant pack) and end up throwing it away because you fed up with it as there are too much of it. As for saving on gas, most of the time gas price in Costco is the same as Arco or some of the cash price gas station.
My 2 cents.
I joined slickdeals to save money but on the contrary, I spend money on things I don't even need and eventually end up in Goodwill or the trash.