My experience with Costco's customer service is so great that I usually will try to buy at Costco if the item is similarly priced elsewhere. They have taken back things pretty much no questions asked, even on occasion produce that rots before the sell by date and before we open. I highly value stores that do this instead of making you fight for a return, even when you have a receipt. We don't return things often, but when we have had to utilize the service, it has been a great experience. I was also happy with how they handled their frozen produce recall last year. I believe I got an instant text message alerting me to the fact that I had purchased items that had been recalled. This is certainly an advantage to their system that tracks all of your purchases.
We get most of our groceries at Costco so we are at Costco at least once per week. We try to buy organic where possible and Costco's prices are hard to beat on most of the organic offerings (at least for our location).
Our weekly staples consist mainly of a rotisserie chicken, one dozen organic eggs (about $2 cheaper than our alternative stores), organic spinach, organic 3 pack of cucumbers and 2.5 pounds of mushrooms. Often will buy multiple chickens and freeze the meat (we use for soups, enchiladas, and salads).
Other items we routinely buy are organic baby carrots, Kirkland organic peanut butter, Kirkland almond butter, turkey bacon, organic quinoa, organic black beans (8 cans for $6; about $2 cheaper), Heinz organic ketchup, organic frozen veggies (green beans, broccoli, cauliflower), paper towels (not cheaper than clipping coupons/good sales, but the Kirkland brand is great), toilet paper, Nature's Domain dog food (about $20 cheaper than the last vet recommended brand of dog food and cleared up our dog's skin condition), organic Kirkland olive oil, organic hummus, Kirkland cheddar, two pack of Chevre, Kirkland Organic Salsa, Kirkland Organic tortilla chips (gigantic bag for ~$5), Kirkland spices (salt, pepper, and all others), Dave's Killer Bread (usually freeze one loaf and eat the other, about same price as buying 1 loaf elsewhere), Udi's bread, cottage cheese, stamps, pork chops (about $2/pound), La Croix (about $0.33 cents a can), organic butter, Kirkland brand coffee, Kirkland multi vitamins and raw nuts (for oatmeal and baking). Seasonal items we usually get our ham, turkey, baked rolls and pies from Costco for family meals.
We also get our disposable contacts, eye exams ($55 for an eye exam, I believe), prescription meds, pet meds (including preventative heartworm meds) and gas at Costco.
Gift cards can also be a great deal. For example, one of the restaurants we love sells discounted gift cards through Costco. You pay $80 for $100 worth of gift cards. So makes one meal free. They have lots of options and they vary as to each location and city, but would recommend checking this out to see if you may be able to utilize for any cost savings.
My tires this year were about $150 cheaper than the competitors. I waited until they had a Michelin $70 rebate, which rotates with Bridgestone. For what its worth, they fill up with Nitrogen instead of regular air. I don't place much value on this, but its an additional perk I didn't see mentioned.
If we want an enlarged photo, we usually go through Costco. They also refill ink cartridges for $7.99.
Oddly enough, the pharmacy now does vaccines. I was able to get my flu shot for free this year (was $12 out of pocket if not covered by insurance). Not a huge savings, but one less stop I had to make.
We occasionally buy fruit/veggies, but not often and usually only the organic berries when in season.
Since we travel hack, we don't book through Costco, but rental car rates through Costco are often the best and hard to beat. I often check this as a baseline. Costco's Hyatt Rate is also often better than going through Hyatt and other sites. You can book a Hyatt room directly through Costco, as opposed to a vacation package. For most other hotels brands through Costco, I believe you have to book a vacation package. I often get the Hyatt rate from Costco, then book directly through Hyatt over the phone to maximize the savings and get the points/utilize my Hyatt status.