it WILL improve your balance, but most impotantly, it will make you keenly aware of the location of each of your limbs at all times. Thats the most important benfit from my perspective, and the muscle toning and strengthening are just an added bonus. I cant begin to explain how useful I think it is.
Thats what I use it to train for, thats an old clip but it shows the arm movement used, and its very similar when on the slackline.