I have one, but seldom use it any more. Instead, i use the handle of my knife to push start the ball. then take a short hold on the ramrod to get it going down the bore. That is actually what I came up with when I started muzzle loading and had not seen nor heard of a ball starter; it worked well for me for 20+ years before I finally bought one.
It's just like a ball puller. I have had one since I started this game, and I used it a couple of times before learning that I could just dribble some powder into the touch hole, or pull the nipple and do the same on a percussion gun, and shoot the ball out. Much easier and puts less strain on the ramrod.
~Kees~