Basically, when I buy something at a regular store, I'm shocked at the recent inflation.
But at Costco, most things have barely budged at all in the past few years. And that's starting at a lower base as well - at the moment it's simply nuts how much cheaper things are there.
In fact, if you look at their financial statements, their entire profit equals their membership fee income - this means you're buying things at cost price on average. And Costco is known to be brutal to their suppliers, using their market position to force lower prices and larger pack sizes onto the manufacturers.
They don't sell everything though - there are some items you have to buy on Amazon or a regular grocery store. But almost (not entirely) every time that Costco sells something, it's the best deal you can get.
Another downside is you have to buy a LOT of what you're buying. So it only makes sense if you're planning ahead somewhat, and if you have room in your fridge/freezer/garage.
EDIT: actually the main drawback is the hugeness and remoteness of the store, and the immense number of other shoppers. But again, this plays into going every couple of weeks and buying ahead. These things are inherent in their model and I accept it. I actually spend less total time shopping now because I do all of it in 60-80 minutes every two weeks.
Another edit: none of this means that the quality is any lower. Somehow, they're delivering excellent meat, produce, and more at a fraction of the cost.
Maybe I should buy shares in this company. They barely have a profit margin though.