I ran one of the BB-mount ones for a while, it does its job keeping the chain on, but it does hang down far enough to hit logs and rocks too high to roll or jump over. Hit it hard enough and it'll slip and jam the chain against the chainstay. That made it a pain where I usually ride (we have lots of logs, not all of which I can jump over cleanly), so I took it off after a while. Too bad the Stinger and my log-hopping skill didn't mesh, it made a big difference in the fast and rough stuff.