I have used them quite a bit. Mostly in shotguns, for sporting clays use. They do add weight. I also had one in a Rem700 375 H&H. Helped balance the heavier contour barrel that was on it.
As ducksoup said, a GOOD recoil pad will help with recoil control a lot also. Put the two together and you will have a much more plesant experience.
If you are putting it in a synthetic stock make sure you have a good solid mounting for it. It will not do you any good to be bouncing around in the stock. It must be firmly (surrounded) and tightly (end to end) positioned so when the gun begins to recoil it is working against the weight of the recoil reducer, not just bouncing it around in the stock.
I usually put a piece of same diameter heater hose in the hole as a spacer, cut just a hair longer than the stock hole with the reducer in it. It would give just enough compression to hold it tight when the butt plate or reciol pad was installed. Also, you can wrap electrical tape around the outside at each end to make sure it fits better in the hole/space in the stock.
B