Lab, the problem with ellipticals is that the cheap ones are, well, cheap. For a really smooth ride, you'll probably need to shell out $2000-$3000 (US) for one of the industrial health club ellipticals. I did a ton of research on this and tried a few of the cheaper ellipticals out at Sears, and there is a huge difference between the ones you can get sub-$1000, and the smooth as butter ones at my gym. The stride length of sub-$1000 tends to be 18 inches or less, which is difficult for the knees. Gym ellipticals generally have a stride-length of 22 inches, which results in a longer step and less knee bending. Unless you're short, the difference in stride lengths is noticeable and a bit painful after a while, imho.
