If done properly they are better for your knees than regular squats. The reason being that when you sit back onto a box yoiur knees never go in front of your ankle this creates less shearing force on the knees than most peoples regular squat form. Back problems only arise if you are doing them wrong and are bouncing off the box or do not stay tight on the box. THey are performed by actually sitting on the box for a split second instead of touch and go like most people think.