An exercise ball is tough because it demands dynamic balance. You really work out your abs. You can't slouch because you'll fall off. I tried it for a couple months and gave it up. If you are seriously considering using an exercise ball, get a puncture-resistant one from a company like Fitter First (that guarantees against explosive decompression).
I will forever associate exercise-ball chairs with the hilarious moment when, in the middle of one of Leo Laporte's podcasts, there was this BANG sound off-mic and he went off the air for a few seconds. Then we heard his voice, a bit distant, as he explained that he was picking himself up off the floor because his chair had just exploded.
Of course, if you land wrong after such an incident it won't be quite so funny.