870, no. 590, yes. I have one on mine (the folding CopStock, not the new SpecOps stock which looks similar).
The recoil reduction thing works. Still bangs up your shoulder because it's a big U shaped metal rod for a buttstock, but the spring mechanism really does eat up the kinetic energy. I got used to it, I guess, and fired a friend's 590 almost identical to mine but with the stock... stock. And the bugger hurt.
So I'm sold on that front.
My folding mechanism's become a little sloppier than it was when it was new, but it still locks up and isn't going anywhere for any reason other than pinching it to deploy or undeploy. I haven't managed to break it yet, which is more than I can say for other parts on that shotgun.
On the 590 the pistol grip sort of gets in the way of the forearm release, but the 870 has it in front of the trigger guard so this should be a non-issue.