I'd say 10% of orders (I order ~2 times a week) contain an item that wasn't of sufficient quality. Their customer support is great when this happens, always refunding the bad item (though sometimes that's besides the point -- I needed that item for my meal!)

The $100 yearly membership fee amortizes to ~$1/order, and the markup on products seems to be ~5-10%. So I'm paying a little extra to avoid going to multiple grocery stores on a weekend/weeknight