My wife is a member at Costco and we are generally happy with this. We have recently experienced a couple of the minor hazards of buying their bulk flours. I'm the baker in the family so when I put flour on the shopping list my wife has sensibly tended to shop price.
1) A 25 pound bag of all purpose flour was bleached rather than the unbleached, which is what he have customarily bought. The flour was also not sifted like the more expensive commercial flours like Pillsbury. This was no big deal, but my perception is that unbleached flour is healthier, so I prefer to pay the extra for unbleached.
2) Next time she bought 20 pounds of all purpose flour that is actually a kind of variant on the all purpose flour theme. It's got fiber and a different kind of grain that is claimed to be more nutritious. That was a good concept but it really alters the breads I make with it, so I am essentially making a new kind of bread. So if you buy flour there, check the details of the label.
As a result of this, I'm beginning to specify brand names on the shopping list.