I've found they help a little ... tried a CH Research magazine cap type recoil reducer on a Beretta 391, together with one in the buttstock - on a lady friends gun as she was initially complaining about recoil when she first started shooting. The two units kept the gun "in balance" with added weight in two places, and from her perspective, seemed to diminish the felt recoil ... perhaps by both adding a few ounces to the gun and by further spreading the recoil sensation out over a slightly longer interval.
Yes, I think they work ... & they are relatively expensive.
Good gun fit, technique, coaching & consistant mounting with appropriate target loads are a better way to approach things IMHO.
Same lady no longer uses the recoil reducers, and has not mentioned recoil
over the past couple of years now.