I am one to definitely say its not a gimmick. While I don't think it helps accuracy much, you can't deny the vibration and sound dampening qualities of them. I just ordered a custom one for my Allegiance from a guy on AT. His look tons better than the original STS and work as good. Uses the same stopper as the STS as well as the bowjax on the the rod.
Plus, like mentioned, if it helps keep the bowstring away from your arm with heavy clothes on, even better.
