Elliptical machines place your feet at a very wide stance, and especially for women, the stride can be quite long. So they can be pretty hard on your joints.

Impact isn't a bad thing - especially not the light impact you get from walking. It helps build bones. There was a study published a few months back that found that even young male cyclists have bone loss if it's the only cardio they do.

Then there are all the health benefits of sunshine, not to mention the mental stimulation of being outside with varying scenery and fresh air.

If I must use a cardio machine, the elliptical is the one I use, because it is a more natural movement than a stair climber, the stride length bothers me less than the excessively long crankarms on stationary bikes (never mind those awful saddles! ), and the treadmill is just too-too boring. But I avoid them if possible.