Worth it:
Kirkland baby wipes (pre packaged and come with travel packs now)
Diapers on sale ($30 for like 96 pull ups)
Kirkland coffee (tasty and a best buy, especially K-Cups. When it goes on sale I buy 3 months worth)
Any non-perishables are usually better buys.
Kirkland detergent and fabric softener.
Outdoor stuff: Hoses, seed, and fertilizer.
Pool stuff: chlorine and shock
Milk: $1 less on regular milk and the organic 1.5 gallon pack is cheaper then just 1 regular gallon of the organic.
BACON!!!
Hit or miss:
Meats. Sometimes they are better then regular grocer prices, sometimes not. I find their NY Strips to be the best buy at 7.99/lb compared to the local 10.99/lb at the grocer. Chicken is different and even the bulk freezer pack stuff doesn't beat the plain old Tyson chicken breasts. Plus the Tyson stuff is better trimmed.
Paper and office supplies are often a little higher then the sales at Staples but I usually grab them anyway because they $1-2 more in the bill isn't worth extra stops.
Heat-at-home meals (Soup, chicken pot pie, enchilada, and salmon are good. Stuffed peppers are OK. Everything else has been sub par)
Computers. Sometimes a good value, other times not.
Don't buy:
"fresh" Breads. Stale after a day and they are very chewy in between. If you get it fresh and eat it right away they are just so-so.
Beer. Unless on sale I skip it. They are usually at grocer store levels for price and styles. The only exception is their holiday beer brews from Ommegang which is 2/3rds the price of regular plus it comes with a nice glass.