I love the Chuckit!  Let's me throw a ball MUCH farther, for my fetch-crazy dogs, than I can throw by hand!  Also saves me from having to pick up slimy wet balls, since my dogs will drop them at my feet and I just pop the throwing stick over the ball and heave it again.  These replacement balls work perfectly with the throwing stick - my only complaint is that the balls are not as durable as real tennis balls.  The balls that came with the original throwing stick split in half under the abuse my Cattle Dog gave them.  Now he's a senior citizen and has no interest in fetching - so maybe these balls will last longer with the other two Aussie-mix dogs.