Got these some years ago.
They work, you loose most rattling and string sounds slightly drier.
Not fit for playing flageolet, but you can just shuv it over the comb for that.
However, the elastic band is kind of cheap and didn't hold out for too long. They still worked, but i had to keep fussing with them all the time.
Now recently i got me the Harley Benton version of these (A proprietary Thomann brand? I'm not sure but i think it is... ?).
I will have to see how long they last, but they already have a more sturdy construction principle, keeping the damper part separate from the strap/elastic part. I THINK they're gonna last longer, they work fine, and they cost half.
Well that was easy..