The holder is a good idea. It's one of those things that seems so obvious, you wonder why it hadn't been done before. But there is a basic flaw with the way it's been designed and made --- it doesn't work anywhere as well as it could or should have done. I have uploaded a photo to accompany this review so that you can see the problems that I identified.
The large image shows the actual design of the holder. The small section, underneath, is what you slide between strings at the headstock and then rotate to hold the item in position. As you can see, the openings at the outer edges tend to widen and this is what causes the first problem.
Ditto for the upper part which fails to grasp thin picks effectively. If you use medium thickness picks, especially if they're not very shiny, you might be OK. But thin picks are barely grasped at all. Very thick picks wouldn't be held either.
The other images show the holder on the headstocks of two different types of guitar - an acoustic and a classical. In both cases, the holder needs to be pushed down near to the nut where the closeness of the strings holds it in place. Slide it further away from the nut and it can easily vibrate and revolve and fall off!!!
The upper right image shows the unit in position from another angle.
So - is it a better solution than threading your plectrums between the strings behind your headstock nut? No - not really.
Is it worth buying and trying? Possibly, but only if you use medium picks and still keep a couple in your pocket for emergencies. Because the holder may well work for some guitars and for some types of plectrum, I've awarded it 2 stars - but it was a hard call and I nearly gave it just 1.