Front squats are great.  Hits the quads much more and much less stressful on the back.  That being said they are a very uncomfortable for most to do.  Hence they are not done nearly as much as regular squats.  
i find that they require less flexibility in my case than a perfect full back squat
to me the reason is obvious
becayse you have the barbell in front, your torso i more upright, that means your hips dont have to rotate as much forward to maintain a perfect flat back on the bottom
there for it requires less flexibility than a backsquat
common sense really