I love my exercise ball. I have been using it when I sew for years. I even let the air out and take it with me when I leave for the winter. Of course I take my sewing machine and a box of fabric, too.
The ball that I have is a heavy rubber, purchased through my Chiropractor, not the soft ones that dimple when you sit on it.
Balance is no problem unless I cross my legs--oops. I use to have lower back problems, but have improved greatly and no fatigue from sewing long hours.