I agree Canes, I have bad knees (it runs in the family) and when I do Smith Machine Squats I can feel the strain in my knees. It seems the Smith Machine limits your range of motion front-to-back and forces the knee to absorb pressure that you could normally avoid by shifting your balance.

And Bad Man--if you mean 'has a gym additction ever kept you from properly resting an injury" I gotta say yes! I hurt my elbow and I can't justify taking a break to let it rest. I know I should, but I keep putting it off to later when I know I will be forced to take a break for other reasons.