When I used to board mine, I got my gelding this ball thing that hung from the wall. He never touched it and I eventually just gave it away. Since then, I kind of felt like the OP in that they were a waste of money. Several years ago I was visiting a barn and one of the colts had a stall ball. He was a complete ham with it. You could ask him "Where's your ball?" in an excited voice and he would go nuts with it, throwing it around and banging it against the wall. If he was in a paddock you could throw it to him like a dog and he would chase it, pick it up and buck like crazy, and he eventually would bring it back so you could throw it again. He loved his stall ball, and the owner said that if he doesn't have it he paces, paws, and kicks from boredom. That said, it's going to depend on the horse and their individual interest level. I would try the less expensive options that some are suggesting (milk jugs and such). If he likes it, get him something more durable.