Weight machines. Good or bad? Constantly evolving research suggests there are merits and drawbacks of each. Here is some interesting reading courtesy of MensHealth on which machines you should steer well clear of.
Standing-Calf-Raise MachineWhy it's bad: Most men can handle a substantial amount of weight--often more than they can squat--when doing this exercise because of the small range of motion of the calf raise. This adds up to a lot of unnecessary spinal compression, which can lead to severe back problems.
Smith Machine (for squats)Why it's bad: The path the bar follows doesn't allow for proper spine mechanics, which can place excessive stress on the lower back. That's because the natural movement of your body during a squat is an arc, not straight up and down. Stick with the free-weight version.
Pec DeckWhy it's bad: It forces you to abduct your upper arm horizontally--that is, move it away from your body while holding it parallel to the floor--while rotating it backward against a heavy weight. This places tremendous stress on the front portion of your deltoid, which can lead to nerve injuries.