Fist,
I have a bad knee too, and have found that riding my bike to and from work seems to have helped. Its a low impact workout utilizing the full range of motion, it really does seem to help my knee quite a bit.
As far as elliptical machines... I can't do them my legs don't seem to move that way... it just feels too weird and if I try to do a normal pace in one I stumble off the machine. If I force myself to the motion of the machine I can't walk right for a little bit once I get off it, lol
I used to work in a sporting goods store and would try out the machines if it was a slow day... I kinda like the tread climber, but would never pay that much for one. My current exercise regime is one I worked out myself with help from Flora. Thank you Flora! Based for the most part on commons sense bodyweight exercises it uses very little in the way of weights, includes quite a bit of cardio, and I've recently worked in a few martial arts exercises as well.
Now I just have to stick to it, lol