The biggest problem with lunges is that as you fatigue during your workout your form gets sloppy. When you get sloppy you get unbalanced. When you are unbalanced.. you over-extend or you offset your weight. Putting all of your weight on -one- knee is an accident waiting to happen. If you shift slightly and bend your knee in the wrong place you can tear tissue.
I'm not sure who came up with the idea behind lunges because it's not a normal every day movement. Kneel to pick something up, sure. But how often do you extend your leg out, put all your weight on it, then push back?
If you want the best leg workout in a solid motion go Squat > toe lift > squat > toe lift > repeat desired amount. Always break your routine up into sets to get the best results if you can. For me I do 3 x 8 squats and lifts with weight. Rest for 2 min.s in between each set. If you're just starting out and doing unweighted then practice your form and go 3 x 12. If you can do x12 then you need to start adding weight.
... and I seems to have started lecture mode. My bad!