It's ok. They run a tour every 15 or 20 minutes all day. As such, there is always a tour ahead of your group, and another right behind you, as the guides herd their groups to each spot.
Visitors are assigned to a time slot with a group of about 25-30. On a weekday in early June, it was about a 45 minute wait until our tour began.
The tour guide was very enthusiastic and animated, and it was entertaining, even though it's more about physics than mystery.
The site contains various optical illusions that make it seem as if people and objects defy gravity - balls roll backward, etc.
Visitors walk up a fairly steep trail to some designated spots that appear level, but in fact, are not. This creates the illusion of height differences depending on the side each volunteer stands on.
Further up, the group enters a small shack built at a steep angle. This creates the illusion of a gravitational force as the body naturally leans downhill.
Children and adults seemed to enjoy it. The tour itself lasts about 35-40 minutes. I visited a mystery spot in California as a child, and naturally, it was a lot more fun (when I believed it was a location with a mysterious gravitational force) than now as an adult.
It was pleasant enough, but there are so many more impressive things to do and see in the Black Hills area. If you're willing to throw down $20 per person for a family activity, your money would probably be better spent at one of the nearby cave or mine tours, adventure parks, or that bear country park.
Other visitors really raved about this place, though, so if you like illusions, or you think your kids will be wowed by them, then this is a fairly quick activity that can be squeezed into less than half a day.