My kitties use the scratcher part but are unfortunately not interested in the trackball. It's disappointing to me, but it is certainly no fault of, or reflection on, the product itself.
Most cats I've seen/known do indeed enjoy playing with these types of toys; therefore, I'm still giving this a 4 out of 5 star review. (If my dumdum cats eventually decide to take an interest in the ball part of it, I'll return to update my rating.)
10/25/2019 UPDATE: I have added the 5th star now because one of my kitties has FINALLY decided to whack the ball around occasionally!