I guess the good thing about it is that it can force you to take regular breaks, even if the break consists of just sitting down.
By the end of the day, my feet were a little sore and I was a little more tired than otherwise, but I felt more active while I was doing it. I would like to try it again before committing to a new desk, however. Now that I've been reminded of it, I'm already thinking of how I can try that at my place.
I keep wondering if I could rig up a work environment where I'm lying flat on my back, on the floor, with an LCD panel mounted far enough above my head that I won't hit it when I get up.
The big show-stopper, as far as I can see, is that I can't touch-type so I'd have to mount a keyboard somewhere roughly above my neck or chest where I could see it... but then I think my arms would get tired quickly.
I've got the 72" version of this: http://anthro.com/cpage.aspx?ssid=12
Their stuff is very, very solid.
Incidentally, being able to experiment with your work environment like this is yet another reason not to work in a 'normal' job. Not too many offices would let you get away with a standing desk, and if your desk is publicly visible the social cost of being the only one standing could be significant.