Treadmills are one of the easiest ways to exercise because they dont require any new learned skill.
Walking or running on a treadmill is easy on your joints because the elevated deck helps absorb alot of the impact.
Because it is a weight bearing exercise it is very good for building bone density.
If you are training as a runner a treadmill can provide an alternative to running outdoors when the weather is too hot, too cold or slippery.
Treadmills come in a variety of sizes. Typically the more users and the weight of those users will effect the price point. The heavier the use, the more robust it needs to be. For instance if a treadmill is being used for running you would typically chose a longer running surface for a runner's longer stride.
Landice and Bodtguard probably have the better better cushioning sytems built into their bed and deck than most treadmills but Spirit's line of treadmills are very good value for the price point and worth considering.