Allan, Don't use em myself, but they seem to work well at Mallens. A lot of hunters do use them successfully.
For some reason they do not seem to work on Julys or collies. As long as you're not hauling either breed, it should be fine.
Are you coming for the Brunswick?

Most everyone is now using them at Mallen's. I finally have converted and am using mine. They won't help much in the first bit after catchup but in the end they are OK.