The same fate seems to have fallen all reverse velocity concepts, including stop rotors. I think the dynamics are just too impractical. Ideally you need active blade twist on rotors which are very stiff in bending.
The ideal seems to be counterrotating rotors which allow the retreating blades to aerodynamically disappear. X2 has taken this approach, and stands a realistic chance of being successful.
I can see the logic in Slowrotor's fixed wing based VTOL approach, with high disk loaded rotors for landing. Ultimately i think the X2 based designs will gain high enough efficiency that even this approach will be questionable.