If you read Technical Tennis you'll discover that all they do is stop string ping, they don't dampen. They also stop a strings natural vibration, which is why Federer dosen't use one. It's a myth that has been around for years. The best dampener is your hand, believe it or not. And IMO, those string dampeners you speak of look horrendous. What a gimmick.
Wilson nPro Open
Luxilon Adrenaline 16L @ 55lbs