Have 2 of them, one on my sons Ithaca 37 12 gauge(they don't make it for an Ithaca anymore), and one on my daughters pre 77 Remington 1100 in 20 gauge. Have used them successfully for years, and they hold zero with the smoothbore slug barrels and Foster slugs. I personally believe they are much better than a saddle mount, as you can remove the top piece the scope is mounted on and put on a bird barrel and you're good to go. Put the slug barrel on the following day, mount the scope and again, you're good to go. Have never had a problem with them, and have never failed to hold zero for us.
I've even played with my daughters gun, and it holds zero even if the mounting screws for the top piece are not tight. Because the top piece fits into a machined groove on the side piece, the mounting screws for the top piece have to be REAL loose for it to wobble. I wouldn't hesitate to buy another one.