I had one with my first pregnancy and used it when I started having contractions. I would walk for a little while then when I got tired I'd watch tv and bounce on the ball. I think it helped. I did that for a few hours then when I got to the hospital I was 5cm.
It doesn't help bring on labor. But it's helpful during labor if your planning on a natural birth. You can do work outs on it to help during labor too.