While we don't make a point of searching out KOA's, we do prefer them if one is convenient to our overnite stop. Never have we had a really bad KOA. Many is the time we have ended up in a really sketchy independently owned park, even when they have shown good ratings and been vetted by our favorite "club".
I will say that there was one KOA where the clerk was totally inept, being that it was her first day and the reservation system kept droping offline. Eventually an experienced member showed up and forced her way through the system and got us taken care of. We felt very bad for the newby and extended all of our empathy so as not to make her feel worse than she already did!