I love Costco. But I wouldn't shop there thinking it's definitely going to save money. Don't get me wrong, their prices are almost always better than their competitors, but you have to compare apples to apples (and be sure you're going to use the entirety of the bulk that you just bought). For example, you can buy a kilogram of top quality Parmigiano Reggiano for $20 from Costco, or 300 grams for $11.00 from Loblaws. Clearly it's cheaper from Costco, but only if you actually consume the entire kilogram. At the same time, I could probably get some bullshit "Parmigiano" from No Frills for $4.00 for 300 grams, but the quality will be crap by comparison.
I like Costco because the quality of everything is consistently good (if not great). I also like that they pay all of their employees a living wage. I've rarely come across a cashier that was miserable working at Costco, usually they want to chat you up. If I'm ever unhappy with anything, then I can return it for a full refund with no questions asked (A friend returned a vacuum cleaner that they owned for a year and broke by trying to suck up a sock... And they still got their money back). As long as you keep your wits about you, you can usually save some money, but it's not as easy as some might think.
If you're going to get a Costco Membership, get their MasterCard as well. There's no annual fee. You get 3% back on all restaurant purchases, 2% on all gasoline purchases, and 1% back on everything else (there's no max to this). The money comes back to you as a Costco gift card, but if you don't spend the entire gift in one shot, then they give you the difference back in cash. Last year we got around $800 back in a Costco gift card, and I only spent $150 on my trip the Costco (so the rest was given back in cash). There's also other more standard credit card benefits (travel insurance etc.,.).