If you do a Pow Wow search I wrote a review on them a little while back.
They are absolutely ACE for Hill bows. 'Just' enough to take away any twang from a bare string.
This applies for any bow that's already fairly quiet with a bare string. Definitely my favorite silencer for Hills, r/d and other longbows.
On more aggressive designs, fast hybrids, recurves, they are good if all you need is a little dampening too, especially if you want to preserve max performance (they weigh next to nothing) If you have a bow that's just naturally louder despite brace height adjustments then I think rubber whiskers are better.
They are so light that on some bows you can do two pairs easily. A bag of 4 sets still weighs less than a single pair of rubber whiskers! Crazy.
I love em. They are durable as all get out, and its SO easy to tune with em, if your string doesn't have excess twists you can slide em back and forth just lickety split while the bow is strung to find the sweet spot.
IMHO they deserve an archery product design award of some kind LOL.