I buy the members mark formula ($20 for a tub bigger then any other brand), members mark diapers ($10 cheaper then huggies at sams) and huggies wipes (720 count with tub for $15). I'm not picky on brand as long as quality is there and after 4 kids, I have tested them all out, with proof that sams brand is worth it.
I also buy meat in bulk and freeze in smaller quantities at home... HUGE savings, especially when they have a "Special buy" item.
We buy most of our groceries at sams because we go through so much and its cheaper to buy in bulk. EX: String cheese, yogurt (although I think that if I shopped at a regular grocery store more often, I'd save a few cents on the yoplait i buy), deli meat, kids snacks, koolaid pouches, toilet paper, health and beauty (although i'm with you about not buying 2-600count packs of tylenol, because they go to waste before exp date, but kids motrin and cold medicine is cheap there), vitamins, contact solution, eye exams!, birthday cake and fruit. I have never seen bigger strawberries then the ones sams sells in season. They are the size of apples and delicious. And they don't go bad after a day like the ones from a farmers market...
We also pump our gas at sams club- they always have lowest price (not low price with CASH or COMPANY CARD special), so I dont waste time driving around town looking for a deal or filling up every 2 days. I pump once every other week when I shop and done.
We also like their ice cream sundae at the snack bar :-) We share one for all of us- what a treat at $1.50!
I've found that if you break down per item cost between walmart and sams club, you pay a bit more at walmart then sams. Although at Sams, the initial investment is larger then walmart...
We pay $40 for our annual membership, but save thousands annually on our purchases.