I've tried this and for usual tasks it's fine but for tasks that require deep concentration it was inhibiting cognition
That was exactly my experience too. Programming + standing was neigh impossible.
It's incredibly distracting, I couldn't imagine trying to work on a treadmill, I can't even manage to play turn based RPGs on the treadmill on the lowest setting. The only treadmill activities that have worked for me are TV and podcasts.