I don't know how anyone can stand (pun intended) working at a purely standing desk. You really need a treadmill to make it effective, both for your health and comfort. Standing stationary for long hours is not good for the circulation in your legs. Walking, on the other hand, is much more comfortable and leaves you with a warm after-workout feel after a long day.
Standing is much better for leg circulation than sitting, but I'll grant you it's not as good as walking.
The thing is for me that I don't stand stationary, I move from foot to foot, sometimes kick them around, and pace around the room while thinking. It wouldn't work so well if I were in a cubicle though.
