I have the Side-Saddle and tactical light from Tac-Star, but not the sling. Both have worked very well for me.
You may find that the shells tend to slip out of the carrier after firing several shots. But for me, it takes more shots than the 7 in the mag before the shells in the SS threaten to fall completely out. That's annoying when shooting 100 round practice drills, but shouldn't be much of a concern in a defense situation.
From reading your other post, your gun's forearm (cylindrical tube, with deep ridges around the circumference?) is probably OK with the Side-Saddle. To check it, rack the action all the way back, and see if any of the forearm comes back far enough to cover the receiver. If it does, you can buy a shortened fore-arm , which I think is standard for Mossberg's defense oriented guns.
Tac-Star is a well-established company with a fine reputation.
[This message has been edited by boing (edited November 05, 1999).]