Not all hams like it. Some hams don't mind it, and the balls can make a good substitute when a playpen isn't available. As a rule of thumb, don't force them to go in if they dont want to. When letting them loose in a room like livingroom, they might bump into things at first, but as long as you don't move furniture around, they tend to learn where things are and will bump less (at least from what ive seen).
When they want out they will either stop in one spot for a while, or scratch the plastic.