Is working the elliptical as good as running?

What do you think? What is the relationship between using the elliptical and running? Is there a "conversion factor"? X miles on the elliptical equals Y miles on the road? What about aerobic work content? Equals? Is the elliptical better or worse on the legs?

Feel free to leave me your thoughts.

In the meantime, I seem to have increased my cadence on the elliptical I am using at the beach. I am working at about a 10-20% increase, compared to the elliptical I use at home, measured by 3 factors: 1) elliptical reported mileage, 2) elliptical reported calories burned, and the most important 3) the "sweat factor", measured in gallons generated.

Running here has been a blast, easy and relaxed.