i use the smith machine for squats cause there is no squat racks in the gym where i work out...
now the other gym has them, but everything else is so crappy there! and they close at the time i can workout... so... have nochoise damn it!
I sometimes use them to do shrugs with my hands behind my ass. Just makes me feel a little safer in case I drop it. Other than that, they suck for squats, bench press, and most other shit b/c they are limited to a certain range of motion.
Avoid heavy smith machine squats at all cost it's not a natural ROM and it will fuck your knees up, I was repping 405 on them for a while and then my knees starting killing me, so I went back to the free squat and now no problems