I've used the elliptical often when injured, and maintained the bulk of my fitness as a result.
As other poster said, focus on time and intensity, not distance as distance is meaningless. I like the AMT elliptical as they have the greatest rancor of motion and allow me to come closest to my running motion.
Unlike the other poster, I don't come anywhere 200 spm. I'm usually at about 114. That might be a result of me using a different machine that comes closer to a running motion rather than a stepping motion. If I go any faster at that stride length, I feel like I'm wrecking the machine.
I also don't hold rails, again because I'm trying to emulate a running motion as much as possible. So my arms are on the handles, but I do try to emphasize using my legs more than my arms.
I also agree that you can do more work on the elliptical than on the roads. Use the built in hr monitor to check your effort levels. I find that I can do 6 or 7 intervals of 5:00 in duration near my max heart rate as compare to 4, maybe 5 on the track. I also find that I recover more quickly on the elliptical than on the track - maybe 2-3 minutes after a 5 minute max heart rate interval instead of 3-4. I can also go at tempo pace (90% of max heart rate) for 30-40 minutes on the elliptical.
In any event, I think it's a great tool.