When subjects exercise at the same RPE (perceived effort) on TM (treadmill) or EL elliptical), oxygen consumption and energy expenditure are similar in spite of a higher heart rate on the EL. These data indicate that during cross training or noncompetition-specific exercise, an elliptical device is an acceptable alternative to a treadmill. Comparison of Energy Expenditure on a Treadmill vs. an Elliptical Device at a Self-Selected Exercise Intensity
Check out this testimonial on elliptical training by Marc Davis, a former elite runner and now a coach.
MY COMMENT: I'll be honest- I don't like the feel of training on an elliptical, and I've seen people who can't run a mile nonstop go for an hour on one. If given a choice, I'll use a bike.