Where I work, two people have built treadmill desks. They both have iMacs with extra monitors, so they built out full sized desks. They are both very quiet and not disruptive to the other people nearby. The keyboards are louder than the treadmills.
The employees at my work that have treadmill desks tend to rock out a slow, steady pace. They don't really work up a sweat, but the long term, lower impact movement is far better than sitting or standing all day. I would think you still need to work higher impact workouts into your week at least 2-3 times. I also think there are workouts that are essential that require 100% of your focus (such as yoga) that could never be replaced by this.
I have a standing desk at my work, because they will convert any desk into a standing one at the employee's request. Many people have it, but we have sitting in all our conference rooms - I would love to see standing set ups in some of the conference rooms as well.