I put a similar product on all three axles of my crane truck last year
http://www.centramatic.com/balancers.rhtml and am very happy with the results. I did a lot of research first, and other then the initial cost, heard nothing bad. Exact same concept on the prop, makes perfect sense to me, the weight would be the only drawback, screw the cost it's only airplane money.
5 minutes later.....bought one, shipping today! And I had a good conversation with the company rep, they are looking for feedback from a Rotax install and I told him I would give him an earful. I will also blab the results around the Rotax crowd.
Friday I had $600.00 worth of front end alignment work done on my crane truck, then after that I had my front tires ($900.00 each) broke down and swapped side to side, just like I do every year with my Airstreaks, NOT just rotated. The tire shop noticed the Centramatic balancers and didn't bother to balance them in the traditional fashion, they said I was good to go as the balancers do a better job then they can!