Silent Subwoofers ie: Buttkicker -your thoughts?

I am a huge movie fan, and have finally completed my theatre room. I have one final question, does anyone out there have any experience with the 'silent subwoofers' such as the Buttkicker made by Berkline. They are generally used to mount to a chair or couch and shake without making any noise. Their purpose is to make your couch feel like it is rumbling when a volcano erupts or when a building blows up.

Are these things for real or are they cheap gimicks that are useless???