Do they work? Sometimes.

they reduce the vibration/oscillation of the string, so how much they work really depends on the bow and arrow combination and setup. 12 fps is a huge jump, I've more likely been in the 3-5 fps range.

They do however quiet the string down, again depending on the bow. I use to shoot a PSE Mach-8 that was noticeably quieter with them on. I now have a Hoyt ultra-tec and isn't nearly as noticable.