I'm no expert guitarist but I do rather like the feel and certainly the sound ( and the smell ! ) of these picks. They are nice and grippy, and the sound they produce is much richer, far less harsh, than a plastic pick.
On the downside, they are considerably thicker than a conventional pick, which probably will not suit a lot of guitar players, and this can take a bit of getting used to.
Durability is as yet not rateable - I have not had enough use to make any objective comment and for that reason I have only marked 3 stars - but for the moment they seem to be holding up well. However, I am almost certain that in the long run, a traditional material will be longer-lasting; but is that necessarily the aim? I think not.