This exercise is great for beginner and intermediate lifters because it gives them a chance to find their workable range of motion, puts the quads in a great stretch (if you're getting deep enough), and it's knee friendly. Once you've got these mastered, you can move on to traditional sissy squats.
The key is to press the shins into the floor to drive the upper body back into the starting position. Try to keep the torso as straight as possible. With this movement, the torso is the resistance. When you lean forward you're lessening the resistance and making the exercise less effective.
Do 4 sets of as many reps as possible, then try to beat that total rep count every week.