There are actually studies about sitting in a regular chair vs. an exercise ball. Here and here for example. Neither seems to show net benefits. The limitations of both studies are small sample size and short time period of observation.
Anecdotally, I know at least two people who both have done it and they seem happy with it (but this is recent, so I can't ...
Aren't you in luck, I asked a very similar question over in physical fitness SE:
Here is the top answer:
A recent meta-analysis of 23 published studies is MacEwen, MacDonald,
and Burr, "A systematic review of standing and treadmill desks in the