I actually read an article the other day that said not only are smith machine squats ineffective, but they are also DANGEROUS.
From webmd:
5. Squats on the Smith machine. This is a squat you do standing at a machine that has a barbell on a sliding track. The barbell rests on your shoulders, behind your head.
In a true squat -- done as you hold a barbell at your shoulders -- the bar doesn't go straight up and down as it does with the Smith machine, Warpeha says: "Looking from the side, the bar has some sway."
"On the machine, the bar doesn't give, so it forces the body into disadvantageous biomechanical positions," he says. People also tend to put their feet further in front of their bodies when doing squats on the machine, which adds to the problem.
Considering that today's adult population is wrought with knee and back problems, says Danberg, the last thing you want to do is an exercise that might aggravate weakness and injury.
http://www.webmd.com/fitness-exercis...tive-exercises