I'm not completely convinced yet.
From the first day it did help my lower back pain. However I think I was trying to sit up too straight and my upper back was hurting even more.
(DON'T arch your back!)
Yesterday I decided just to sit comfortably and make sure I just kept moving a little on the ball. (Rolling it around ever so slightly.)
By the why you can slouch on the ball if you aren't carful not to do that.
So I think after yesterday, my back feels better and it isn't near as stiff when I get up from the desk.
I'm still using it because it's definitely better than the chair I had.
Let me know if anyone else is using one.