I have tried them on a machine Leg Press type thing where you can lower your feet down to the bottom of the plate and get on your toes and perform a sissy squat. Good for stretching out the squats and I used to like supersetting sissy squats (to isolate the quads) with heavy squats...
Good luck!!
sissy's also help bring out quad development at the same time strengthening and stabilizing the core. Try to do them as your last leg workout because you won't be able to do much more thereafter. Sissy squats are not done for long periods of time. 6 weeks on and 6 weeks off will ensure that your knees get ample time to rest.