I find that using a Pilates ball (by that I mean the sticky Mini Fit Ball type size) is one of those love/hate things for backs.
I have a few massage clients at the moment who've been using them in their Pilates classes, and all have aggravated their backs with it.
But I know people here have found the ball to be really useful.
Personally I find that if my back is niggling me at all, using the ball will most likely make it flare up. If my back is feeling great, the ball is great! But you really have to watch your technique.