Have any of you guys tried these things? they say the can give you a few extra FPS and still quiet your bow? or is this just another gimmick out there?

I haven't used them personally but a few of the customers down at the shop have brought bows in with these buggers installed. I cannot really see a benefit to them over something like traditional rubber string silencers or the leeches. I would think it possible that if you had a chronograph and some time then you could find their optimum position on the string but am unsure if this would actually gain you any speed.