The first and worst are the little hydraulic floor models that have a four inch range of motion with one pedal being pushed up as the other goes down. Don't bother with these ones.
The next is the larger, health-club version with the same mechanics of one pedal going up as the other goes down. This is somewhat better as the range of motion is greater. The best version has independently moving pedals. This better simulates actual stair climbing because you must actually raise your leg instead of having the machine force it back up for you.
The last type of stair machine is the treadmill type where a set of stairs continuously rotates around. This most closely approximates actual stair climbing, as you must constantly step up. Be sure to push off forcefully, otherwise you will run into the same problem as in the treadmill where the backward work is done for you.

The stair climber is a low impact machine that can be useful for all levels of fitness.

They only apply force at about 1.2 times your bodyweight, which is comparable to walking. Actual stairs apply forces of twice your bodyweight on the way up and up to seven times on the way down.
Stair machines are not bad for your knees. Those with back problems should be careful not to lean over excessively.
A potential complaint about stair machines is foot numbness. This comes from continuous pressure on the feet for extended periods of time. Lift your feet off every now and then to help circulation.
Do not lean on the rails while stepping. Many trainers tend to lock their arms and support their body on the railings at arm's length. This decreases the effectiveness of the exercise considerably, reducin...