I think the Smith machine gets a unwarranted bad rap. I tried regular squats. My heels tend to come up the lower I get because my Achilles doesn't stretch that far. This causes me to start to lean forward turning my squat in to a big fat good morning at the bottom that hurts my back.
With the Smith machine, I can put my feet slightly forward, so they stay flat the whole time. I can put all the weight on my legs instead of transferring it to my back near the bottom, keeping my back straight throughout the rep. Yes, there is a trade off in not getting the benefit of "handling" the loose bar as you do in a regular squat.
But with training legs, you use large weights and having it shift the wrong way, then trying to correct could wreak havoc if done wrong. Where as the Smith just goes up and down, so no threat of falling over.
After squats in the smith, my legs are sore, but not my lower back. I know, I don't look as cool as the guys who use the squat rack, but I just turned 47 today, have no back problems, and don't want any or anything to curtail training. I yes, I do ass-to-grass on the machine.
Its a great machine to work legs if you can get over the "your not macho" if you don't free squat.