I recently put an Urbino stock on my 870MM, got it from GP Tactical.
I really enjoy the reduced length of pull but I've also discovered that after shooting it, I'm initially not a fan of pistol gripped stocks. I've found that the recoil has been redistributed to my cheek and web of my shooting hand; less comfortable than when the recoil went straight to my shoulder. I've also found that a pistol gripped stock has made manipulation of the gun just a little clumsier, especially when trying to access the action release and reloads from the buttstock mounted shell carrier aren't as smooth as my forearm gets in the way of the shell closest to the receiver. I'd like to run it in a shotgun course before my final verdict. I also dropped quite a bit of coin on it, so for now, I'm going to try to learn to love it.