I used to have one in college and worked out on it all the time ... the motion is almost obscene. (But maybe that's why I liked it? [Snicker, snicker])
Anyhoo ... I found a used one in the paper for, like, 75 bucks and bought it last week. And I am now an air-humping MACHINE, baby.
But seriously: Holy crap, is my lower back sore...