I've been using one of these for the last year and they really do work. When you "holster" your rifle, the rear stock is held close to your body and doesn't swing or flop around when walking/climbing. It makes it much easier to move up and down steep or rough ground where you want two hand for balancing and holding on.
I usually use mine with the daypack attachment. That has the added advantage of leaving the rifle sling with just the right amount of slack for use as a shooting aid.