I exercise a pack of German Shepherds with this.  They love it.  The tennis like ball that came with it lasted about5 throws before they had it chewed in half but the chuckit balls that fit it (I use large size) seem like they will last as long as anything does with those big teeth.  I can give the dogs quite a work out throwing the balls about a hundred yards up a steep hill, in rapid fire if you have enough balls and the dogs are trained to drop them when they return.  So easy and effective and no more dog slime (well, I do have to hose off the launcher and chuckit balls).  Just make sure you get balls to big for your dog to swallow.  I have two friends who each lost a GSD to swallowing a too small ball.