My family just stayed 3 nights at the Spearfish KOA, and I'm happy to report that we saw no traces of the behavior previous posters have reported from the campground hosts. In fact, it was quite the opposite. Upon our arrival, the guy came over and greeted us, pointed out several things in the area to do aside from the obvious attractions, and even brought us his own personal fire ring with a large stack of firewood to use free of charge.
The campground was a great place to make our home base for exploring Devil's Tower, Sturgis, Deadwood, Mt. Rushmore, Crazy Horse, the DC Booth Fish Hatchery, Spearfish Canyon and more. We were just minutes off of I-90 while still being secluded and quiet. There was a good Mexican restaurant within walking distance, and we were within a few minutes' driving distance of several other places.
We had a large pull-through site with plenty of space between us and the neighbors, and plenty of space for an extra car to park. We had a nice, big shade tree and a picnic table, but no fire ring. Only a few of the spaces had fire rings, I suspect because the winds are very high and often there are fire restrictions. The bathrooms were clean and had plenty of hot water. The laundry facilities were a bit overpriced, but nice to have the option after being on the road for several days. The pool was nice, clean and warm.
Overall, we had a great stay and thought this was a great place to base ourselves to explore all that the area has to offer. We thought the grounds were nice and the staff were friendly. If out this way again, we'll definitely stay again.