I have always preferred state and national parks over any private campgrounds. Unfortunately, they aren't always convenient to our chosen route.
KOA's, whether they are the destination rv parks, or the roadside convenience parks, are what they are; gravel parking lots with a pool. But, like stopping at Burger King, you pretty well know what to expect. For the most part they are just places to pull over and rest.
Really, of the 33 we have stopped at over the years, there are only two I would never return to (Little Rock, AR which was a run down trailer park, and Estes Park, CO where you were parked so close you couldn't open your awning). There are even a handful we would recommend, whether for the place itself, or the people running the show. Which, for us, is comparable to our batting average with state/national parks. We haven't been that lucky with other privately owned campgrounds. Granted most of our bad experiences occurred before Al Gore invented the internet. I've pulled in at a couple of places with the smiling Sam logo and wondered which would happen first, a gun shot, or a meth lab explosion.
We have been lucky with KOA, so far. Which is why I didn't understand the whole Keep On Adding thing until I read about other's experiences. We have never been charged more than advertised at a KOA. I am, in sick way, looking forward to the first time it happens. My wife can be entertaining in moments like that.