Alot of variables come into play that will determine just how productive they can be. To increase the effectiveness try things like:
-keep the reps at a slow/controlled speed
-go absolute rockbottomasstothefloor! these are sooo much more effective than just going parallel.
-pre-exhaust....do them LAST. Start with exercises like lunges, extentions, leg curls (if you have the leg attachment on a bench), front squats, stiff-legged deads, to fry the muscles before squatting.
-do only 1 or 2 sets of higher reps. go 20 , 25, or even 50 if you can. Just grind out as many reps as possible, rest while still holding the bar in position and just breath deeply a few times to get your wind then grind out more reps till you hit your magic number.
Good luck
KR