I usually go to Costco once a week and fill up on staples. Paper products, laundry and dishwasher detergents, eggs/egg whites, bottled water, etc. They generally have high quality meats and seafood at reasonable prices, and typically the lowest gas prices in the valley. I have bought a variety of other stuff, from clothes, books, sporting goods, furniture, mattresses, appliances, tires, etc. and have almost always been pleased, and when something doesn't work out they take the item back with no questions asked.
Some times are busier than others, and I try to avoid them, but even when the lines are long they generally move fast and they get you out reasonably quickly. I have their Visa card ( used to be AMEX ), and the cashback I get at the end of the year is multiple times the membership fee.
I don't buy everything from Costco, and when I need to pop in for a small purchase like gallon of milk, loaf of bread, etc. I go elsewhere, but have always found Costco to be good quality at affordable prices. I can see single people not thinking that the trip is worth the hassle, but when buying for a family it is a great place to shop.