I've been using the Elliptical to supplement running for the past couple of years. I've changed things around a fair bit, from doing a 45min CT interval session the evening after a morning session / steady run and then a steady mid-way through the week, to trying to maximise benefit with 2 short sessions of 1min hard / easy for 20mins and 10mins of 30s hard / easy.
My coach and others I've spoken to suggest only using the Elliptical for steady efforts, purely to add volume to training. I do 2 hard sessions a week and a long run, but since I'm only adding about an hour a week of CT, I see the benefit to doing quality work then (getting the HR up and the lungs really working). Obviously the mechanics aren't transferrable, but it's still getting the heart really working hard and it's got to be more beneficial to just a couple of 30min steadies? The proviso for this is that it doesn't affect running training, which can be pretty easily scheduled.
It's amazing that you can do a session only 6 hours or so after a hard session or even race, but that's kind of the whole point in the elliptical right? It doesn't take much out of your legs, it's more the mental challenge.
If you were purely Cross Training you could do a session every day virtually, so why limit it to just steadies? I'll do either, but want to get the most out of it.