The Fernco seal pictured at http://www.fernco.com/plumbing/wax-free-toilet-seal looks exactly like the one I used in one of my rentals three years ago. It worked fine until the bowl had to be replaced because of a broken tank a year later. When I tried to lift the bowl with the wax-free Fernco seal on it, the "flexible ribs" were caught under an internal lip of the floor flange. Because the flange wasn't properly mounted to the subfloor, and the drain line was ABS (black) plastic, when I tried to lift the bowl the drain line flexed and allowed the flange to be lifted.
To free the bowl, it was necessary to lift the bowl (and the flange and drain pipe with it) high enough to get 1-inch wood blocks under the bowl at several places. Then it was possible to use an old carpenter's saw to cut through the Fernco seal sleeve and free the bowl
The Fernco seal looks like a "better" idea, but I did not use another one. When I installed a new bowl I used an "old style " wax gasket.