Smith machine squats and leg extensions work great for me.........my form sucks at free squats.......they make my lower back hurt plus I like the stability of the smith machine.
And I don't really care if free squats work my abs and lower back better.........I've got deadlifts and crunches to do that........I guess it just depends on the look and what your going for.........
I make consistent gains with smith squats and leg extensions alone....so that's what I use............
Peace...