My general rules are:
-Foreign products are often quite good and worth a try(British/Australian cheeses, Italian sparkling drinks, German treats,etc.)
-Meats are now ok but limited in selection (the new vacuum packed beef seems to be sourced from a similar vendor that supplies other large grocers in the area and is perfectly usable)
-Only buy as produce that be used in the short term (4 days or less) as it tends to go bad faster than elsewhere
-Most of their higher end household products (Tandil laundry detergent, Tandil oxi, toilet paper, cat litter) are good but might not be cheaper than Costco (one product is sold as Kirkland Signature and Costco is cheaper on a unit basis)
-Crackers/chips/cookies/etc are very cheap and often very good clones
-Major label items (Coke, Colgate, Head and Shoulders) are only equal to regular supermarket prices and more expensive then when on sale
-Stock up on Special Buys if you find something you love as they might not come back for a long time
Personally, I am a huge fan but I would definitely would give some huge caveats to a first time shopper (quarter for shopping cart, private labels, debit, etc.). Follow the above,use common sense (such as avoiding products/produce that look crappy), and be willing to try new products. I understand its not for everyone as I left the first time without buying anything thinking what is this BS but I have been converted.