We don't typically stay at KOAs but we have on occasion. 3 or 4 years ago while in northern CA we decided to stay at the Petaluma KOA, one of the nicest campgrounds we've stayed at. The staff there was amazing, their SF city tour was awesome!
This last April we stayed at the KOA in Cedar Falls IA, very nice layout, easy to get in and out of and the only campground we've ever stayed at with a gas/diesel station on property! Their WIFI was great but it was early in the season so not too many people staying there. A bit pricey but like most of the others have stated, you usually get what you pay for. It's about choices, you have to stay where YOU'RE comfortable.
Dan