According to this article:
“Ergonomic experts analyze the way we work, and how we can do so more effectively and with less stress. It turns out that the stand-up desks of history are extremely good for reducing injury. Standing doesn’t allow the leaning and slouching that is possible in a seated position, your back and neck remain straighter, and a properly positioned monitor allows users to look straight ahead, minimizing neck movement and strain.”
Basically people aren’t designed to sit for long periods of time. As hunter-gatherers we are supposed to stand and walk or run most of our waking hours. Thus, standing while working is good for most of us.
It goes on to say:
“Working while standing does provide more energy, and eliminates the afternoon doldrums almost entirely – in fact, some proponents use their stand up desks only in the afternoons, finding that sitting through the morning appeals to them.”
The one problem is that if you have been used to sitting all the time instead of standing your feet will bother you. It will be important to wear comfortable, correctly fitting shoes if you plan to spend more time standing.
A number of sites comment that “studies show…standing is better” but I can’t find references to the actual studies. However, sitting is know to put the most stress on the back, and folks who work while standing are more likely to move or shift position, thus encouraging muscle use and discouraging repetitive use injuries.