elliptical trainer workout routine

Familiar with the elliptical or arc Anyone?

I usually drive to the gym involves minute 20 run on the treadmill and 10 minutes on the elliptical. However, I heard that the treadmill is terrible for the knees, especially if you've had injuries knee. I decided at the altar of my routine and do 20 minutes on the elliptical and 10 on the arc trainer, sometimes I'm going to up and do 20 minutes of arc on the trainer and 10 on the elliptical. do you think it is just as effective as the treadmill? I know racing is the best form cardio, but I am worried about the impact it has on my knees.

I use an i (elliptic knees are bad too) and get excellent training there. but here's a site that compares them if you want another opinion http://exercise.about.com/cs/exercisegear/a/treadelliptical.htm

Workout Routines : What Muscles Does the Elliptical Work?