I've talked to a few people and from what I gather, those little mini bikes might be good for helping circulation if you have a health problem, but they would not result in enough calories burned to cause noticeable weight loss.
On the other hand, there was a study published last year that said people who fidget are more likely to be thin. The researcher does all of his work standing on a slow moving treadmill. Read a little more
here. So if you used it enough every day (most of your workday) then you might find some benefit. But it won't replace traditional exercise, and would not provide cardio benefit.
Maybe someone who uses one will share their experience
