Greetings Kindablue, and welcome aboard,
With its low price, I'm sure many are happy with the Knoxx comp unit. But, I question the basic design concept of the system.
With the Knoxx system your shoulder is helped but your cheek will still take full recoil. This may not be a big deal for casual shooting. However, if you're shooting several hundred rounds a day in competition, your cheek might like some relief, too. Have you considered a system where the comb reciprocates with the butt?