I was never a big fan of Smith machine squats, but would be lying if I said I never did them. As the others have said, the natural arc of a free barbell squat is simply not there because a Smith machine bar travels in a fixed straight line. If memory serves me correctly, placing your feet a little further out in front as compared to a regular barbell squat should compensate for that. But you should be able to put barbell pegs on the outside of the Smith machine frame to do free-standing squats. If not, check Craigslist for a squat rack – they’re cheaper than Smith machines!