I've seen them before - probably on Purrs. I quite liked the look of them.
I have two indoor cats who are chipped but not collared, because I had my doubts about the safety of collars.
However these did look safe, and hopefully adaptable - my two are Singapuras with quite small necks; I sometimes agonise over whether I should collar my boys in case they unexpectedly end up outside, or escape en route to vets etc.
However I did get fed up with the fact that though I checked back over a period of time, the site never seemed to get any further forward, or the collars become available for sale.