Having to toss unused bulk food is a problem. On the flip side, Costco has a very liberal return policy, other than for computers/electronics. Virtually no time limit as long as one has a receipt (although they might question something that's five years or more old). Also, food is still generally cheap, and even cheaper in bulk, so if one winds up tossing some (mine goes to my backyard compost pile whenever possible) it's not that big of a financial hit, I think.
The impulse item issue can be problematic - here today, gone next month. Still, some of the items seem to return on a seasonal basis, and often the second time around they are discounted even more, so it can be fun to keep an eye out on an item and then swooping in and scooping it up when it gets even more discounted.
Some Kirkland items I like and regularly purchase: TP, paper towels, undershirts/underwear/crew socks. Cookies (macadamia/white chocolate rocks), romaine lettuce, red bell peppers, carrots, tomatoes (out of season), Colombian coffee.
I've given up on their tennis shoes - the insoles tend to buckle and warp with wear and wind up making the shoes too tight, with the toes point up.
Their jeans, while not as good as name brands, are good enough for work/play and generally very reasonably priced. I liked their black jeans but haven't seen any in the stores for a while now. Usually I try to spot when a new shipment comes in so I can find my particular waist/inseam combination.
Also good: their printer paper, office supplies in general, car batteries. And their meat (beef mainly) is top notch and generally much better quality and same or lower price than what one can get at a typical supermarket. Again, the bulk issue can be a problem but it's a reason to have a chest freezer...
In the stores here, you have to buy two gallons of milk, generally, which is way too much for one person. Their local business center store, however, sells 1/2 gallon cartons of the "Producers" brand, and that is ok. But I also like the 1/2 gallon milk from Lucky or Safeway.