They're good for burning calories and for toning your legs. They give your thighs, buttocks, and calves a good workout. In general, the best exercises are the ones that make you use your whole body, like swimming and running. Elliptical trainers can be just as good if you're pumping your arms and not sitting on a seat. This is what a lot of actresses use when they need to get ready to do a nude scene.
I have had one for 6 years now and I love it! I get just as good a workout from my trainer as I do when I run... but it is much better on my knees. I agree with using the arms... it really tones my upper body as well... I have mine in my living room and sometimes just get on it for an hour or two while watching a movie.... keep up the good work
Something else that's worth trying is a rowing machine. You work your legs, arms, and back muscles. The only downside is you're sitting down and most rowing machines are like bikes in that they have the worst seats.