Re: Using a stair master/stair climber
I would classify any stepping machine as primarily a piece of cardio equipment - the major muscle worked is your heart. Because of this, think of it more as a fat burning machine as opposed to a muscle building machine.
As you are using your legs, every major muscle group below your waist will tone up rather than bulk up. These would include your glutes, quads, hamstrings and calves. If you are overweight, because of the fat burning benefits, healthy improvements would be visible body wide rather than any spot in particular. Remember we cannot tell our body where to burn fat from.
If you do get one, make sure you use it properly and only use any handles for balancing when needed - not for supporting your weight while your legs go through the motions.