I have never used one and am wondering if they really work?

They do some what. They add weight which helps. What they do most is take the sharp spike out of the recoil. This tends to reduce the pain from recoil, which is the thing we like the least. Don't expect the same reduction you would get from a muzzle brake,,,,,Tom

weight for weight, merc tubes are better than the SAME weight of lead for reducing felt recoil

Mercury recoil reducers and lead do reduce recoil and as Jeffe says the mercury works better than the lead of the same weight.

Both can be problematic and destroy the balance of a double rifle. Before installing one tape it under the butt stock and see how much the balance is effected. Recoil is a subjective effect, and more mental than damaging in most cases.

I’m not a fan of changing weight distribution in a double rifle. Best to me is to find a rifle that fits you perfectly, and learn to shoot the recoil as it is. With a quality recoil pad, proper pull length, and balance the recoil is far more comfortable to shoot, but one still is better off to simply become used to the recoil. That IMO is a better line to take than modifying the balance!

Of course that is simply my opinion, and not a mandate to others!

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When building a bolt-action big bore, the balance is often quite different than the starting donor rifle. A recoil reducer might restore balance in those projects if installed in the correct position.
Start with the 8oz reducers for the least disruption.

Save your money for ammo and just add lead shot and epoxy to the butt and forearm to suite your taste. Adding weight does more to reduce felt recoil than anything else. I've tried the mercury recoil reducers and to be honest they made the gun feel awkward in the hand( even when balanced. I have not bothered with them for 10 years now.-Rob