I am very fond of my home-built treadmill desk, but basically never use it any more. Working at home with a toddler running around is hard enough. Working at home with a toddler attempting to run on the treadmill with me is impossible.
Now that I have had one for a while, and before I get rid of it, a few thoughts on the desirability of treadmill desks.
Pluses: I really could stand and work longer than I can sit without taking a break, and it is surely better for me to stand than to sit most of the time. When I used it for actually walking, rather than just standing, the health benefits surely increased and I found it easy to walk very slowly for hours at a time. Another, somewhat off topic, benefit is that when one needs to run one's toddler and the weather outside is Denmark, toddlers can easily be convinced to run on a treadmill for long enough to get some energy out. (Always hold the toddler's hand while she is on the treadmill so that you can prevent falls.) Before I started biking to campus, there were times when this was a useful tactic before my bedtime also.
Minuses: Achieving a deep focus on a topic is not as easy when walking on a treadmill (at least my cheep treadmill) as when sitting or even walking around. Some part of my mind always has to monitor my position on the treadmill so I don't slide off the back, and the slight bouncing makes both typing a reading a bit slower. I occasionally play a game of chess against the computer in the evening, and I can beat it at a much harder setting if I turn the treadmill off. Editing manuscripts, I inevitably end up turning the treadmill off when I get a point I really need to focus on. As a result, I rarely ended up walking, rather than standing, when doing any task that required speed or more than half a mind.