If you're going to do these, do them in a rack and so that the bar rests on the pins at the bottom of each rep. Please do not let your poor patellae take the brunt of stopping and reversing direction without benefit of the hamstring pull, especially with all the extra weight you're bound to be using.
These will be good for quadriceps development and for the actual lockout. The actual hand-off at the sticking point in the squat when the posterior chain turns the lockout over to the quads is another story.
Paul Anderson worked quarter squats down to full squats over a training cycle in what's today called "Progressive Movement Training" or something like that.
Enjoy!