1. I haven't heard of anyone saying that rebounding hurts their knees. In fact, many people, myself included, often jump on their rebounder to do higher impact moves in some workouts. Of course, YMMV. Just be sure to bounce on the balls of your feet rather than on your heels (in Futprime's G-Force, Tracie Long cues to bounce on the heels, but I find that it makes it more of a risk to lock the knees).
2. It can provide an intense workout (Urban Rebounding claims that you get the same intensity working out for 20 minutes on a rebounder as working out for an hour ---times may be off---with regular cardio). You can do sprints to really bump up the intensity as well.
3. The number of rebounding videos is rather limited (Urban Rebounding, Champion Rebounding, Fitprimes "G-Force", a few others that aren't worth mentioning because they are very outdated or not too intense), so quite a few folks (there is a rebounding check-in at Videofitness.com) either rebound to music, or to other workouts. I have some Leslie Sansone workouts (that I normally would NOT use because they are not very intense) that I put on and rebound to. I do, however, modify the moves: a side step becomes a jumping jack, etc.. The floor part of Cathe's MIC works pretty well on a rebounder. It's good to get a rebounder workout or two, or a book, to see what kind of mvoes you can do. I also "invent" some moves of my own!
If you have FitTV, sometimes the Caribbean Workout has a rebounder workout.
