If you are going to do them make sure:
1. You are finishing the ROM, and not doing 1/2 extensions. The VMO is responsible for the final 20 degrees of extension, and this muscle is crucial in knee stabilization (and often underdeveloped in many athletes/trainees).
2. Do them in a unilateral fashion. You can do 12 reps on one leg and then 12 on the other, or you can go back and forth between legs (like you would with dumbbell curls).