I have a genuine Blaser kickstop recoil reducer in my Blaser 500 Jeffery, the Blaser kickstop is a stainless steel cylinder filled with tungsten beads and weighs 600 grams, Blaser claims it reduces recoil by "about 20%" to be honest I can't imagine shooting my 500 without one of these recoil reducers fitted even if it is just for the extra weight. Having the extra weight in the butt actually doesn't make it butt heavy either as the safari barrels on the Blasers are extremely heavy, but this is definitely something you want to be aware of on rifles with a lighter profile barrel you can make them butt heavy and it really throws out the balance and feels weird.
So far my experience with recoil reducers is