I have borrowed guns with similar devices and can say they do work. My main objections are the weight they add, the possibility of the screws loosening, and the cost. They can be used when shooting from a gun down position but seem to be at their best when shooting a pre-mounted gun.
Besides the recoil reduction, the best feature I thought to be the adjustability of the stock. I could adjust the stock as I gained or lost weight or if the thickness of outerwear changed significantly. The negative of this is the more often one changes settings, the easier the settings could change. For "set and forget" I feel a stock fitting is better and much cheaper way to go.
I considered something like this for my wife but having the stock fitted to her ended up to be the better option. Many have used these devices with good results so it really is up to the individual as only they know their needs and wants.
