They are a boutique item for sure.
I bought a used Mach 1 version, but it requires the strength of King Kong to put on and take off, though you are supposed to leave it on the neck once it's on and just slide it down below the nut when not capoing.
The Mach 2 version is supposed to be much easier to use, but I haven't made the leap. (I only buy capos anyway so I can cut them up and make partial capos out of them.)
In the plus category, they make interchangeable pads to take into account different neck radii. They also make pads of different material which is useful. One is very hard, the other has more give to it, which makes it preferable for use on a 12-string, where you have thick and thin strings right next to each other.