Our 10 month old lab is ball obsessed, this thing looked cool, a new, dry tennis ball will go 40'; but even one time it will be damp or moist from the dog's mouth, the mechanism is a set of two spinning, rubber, knurled wheels. Because the wet balls have a lower coefficient of friction, the effective range makes it useless, if the balls have any dog saliva. I noticed the knurled wheels were already getting wear, from just two play sessions. For the price of this unit, it makes absolutely no sense at all. I tried wiping off the balls with a rag, but if they were wet at all, I could get perhaps 15' and sometimes they would not even eject.
If you really want this thing to work, suggest you get a couple of 5 gallon buckets of tennis balls, so that you can have enough dry balls to use. I'm not sure if they would dry out either. I used tennis balls, if you use hard balls, you have to wipe them off. Our puppy did get hit in the eye once, she kind of let out a puppy cry, and was blinking the eye, but shook it off. I would be worried about using a hard ball, you risk injuring your dog.
I had a "Nerf Dog" ball launch gun, but the rubber band broke after about a month. A mechanism like this is less prone to degrade with a "wet" ball. I paid $30 for it at Canadian Tire. I think the "spinning wheel" design of this unit is a fatal design flaw, I would like to find something that would work, our lab loves to play ball, it just gets boring after a while.