We have a LeadSled at our range at camp. It noitceably reduces the felt recoil of the big kickers. One of our guys has a 375 H&H and I personally will attest to the amount of reduction it provides.
What I don't like about it is that for me, it is very time consuming to get re-adjusted after each shot. The sled will and does move with the recoil, causing you to have to do a lot of realigning after a shot. However, this should not be a burden for those who are highly recoil-sensitive and can appreciate the way it "eases the pain"