I only stopped doing them because I lost my balance once and came pretty damn close to destroying my knee.
I guage all leg exercises by how it feels in the muscle and how much energy I have to use to really fatigue the muscle...regular squats spread the resistance out too thin for me. When I want to hit legs, I mean LEGS and not a little bit of legs, little bit of glute, little bit of back...just makes for all smoke and no flame.