Certainly there is some stress on the limbs and knees when using an elliptical, but, as you know, the brute impact of feet hitting the ground with all your body weight on them is missing altogether.As a side note, I wonder how much stress, an elliptical (due to motion of your legs) puts on your knees as compared to swimming?
When I use an elliptical I don't have any knee pain associated with the exercise as compared to when I go for a run, outside or on a treadmill. I don't really run anymore since I left the Marines and will only go for brisk walks outside or on a treadmill.
Swimming is great for the non-impact aspect as well as it is a very good all-body workout too.