Here is a little review I did of a KOA in Boise-Meridian.
http://www.campinnforum.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=28708#Post28708
We have also stayed at the one in Lincoln City, OR (at the coast). You can find that one on my blog as well.
I think as a place to stop for one night to have a shower, doing laundry, etc, they can be great. I am not a great lover of staying at them for much longer than a night for several reasons. 1) you are all packed in quite tightly. We were traveling with our then 4 year old son and the folks next to us helped teach him a very colorful volcabulary. They were nice enough, but we're also there to party. 2) They don't tend to be very scenic, even if they are in scenic areas. 3) They can be pricy compared to alternatives, especially for what you get.
Sometimes they offer just what you need though. The folks at Boise-Meridian were very friendly and made you feel very welcome. They had nice facilities (pool, laundry, showers/bathroom, pool room, TV area, etc). They were close to the freeway and it was a good spot to stop and rest. But it was in an industrial park and you can see from the pictures what we were parked with. The one at Lincoln city was pretty run down, people were not friendly and welcoming, had a decent playground, etc. we stayed there 2 nights and decided we probably would only come back if we had to.
Based on my limited experience, that is what I have found. We have stayed at other little RV resorts (for the showers, laundry, pools) along our big trips, and they have been fantastic. There are several we will go back to if we are in those areas. We always prefer a state park out in our parts, but I understand they really vary by state (ours are really nice, free warm showers, clean, etc so we are spoiled!).