I'd have expected a medium to be a good fit for a 5-string bass, but you have to wrap it around to a point where the Velcro doesn't catch easily. This is on a 19mm first fret, 45mm nut.
Dampening effect, once it sits well, is very good and does what it should. I might get a 'small' at some point, see whether it's any better.
Grossly overpriced, but easy to get and easy to use (when you have the correct size). I'm using it on my guitar now, where it's a better fit, but I don't use it much on guitar. I don't regret buying it, though, and I've got a bass in the post that may have a slightly fatter neck.