I'm not knocking elliptical trainers, cause I've had some great workouts on them; but if it is the only piece of equipment you use and you go hard on it everyday, you could be in for some repetitive motion sorness in your knee joints at some point.
You typically don't have a lot of options where your feet go, so your usually in the same exact motion everytime, everyday and it does begin to wear on your knee ligaments.
After experiencing this first hand at my gym, I bought a gym quality tredmill that provided me with more flexability and options, the knee soreness left and never returned.
Just my experience, and I'm sure genics have something to do with it.