I set up a treadmill desk several years ago, with a regular treadmill, with a board across the hand holds for a keyboard (bungie cords and a wooden stick to hold up the board), facing a wall with adjustable shelves that I put LCD monitors on. I use a track ball instead of a mouse. I really like the setup, even if I may end using it less that I thought and otherwise alternating between standing or sitting on a tall stool. I had some intermittent problems with the treadmill motor early on that made it hard to use and requiring repairs and kind of broke me out of the habit of using it regularly.
I never go much beyond 1.5 mph on it, and more often slower (even 0.5 mph). I probably have never used it for more than four hours in one day or much more than 2-3 miles. Still, when I am using it frequently, I've found walking outside for long distances to be much easier. I can't imagine any research saying it is the same healthwise as a sitting desk -- unless it was by chair manufacturers. :-)
Dr. Levine's work at the Mayo is what inspired me to try it:
http://en.wikipedia.org/wiki/T...
One advantage of a treadmill desk over a standing desk is that you keep your legs moving more so blood is less likely to pool in your legs.
However, I can see that it is not for everyone. I put one together for my wife but she had trouble typing reliably while walking and so just ran it while on calls or watching videos. Otherwise she mainly uses it as a standing desk or also sitting on a tall stool. She would probably have been happier with just keeping the standing desk setup she was using before the treadmill (since you don't have to get up onto an elevated treadmill surface to use those and have more flexibility where you position it).
For mine, I feel like there have been times it has contributed towards knee strain. I think that may be due in part to the limit of treadmills as unnatural walking experiences? One other downside to getting one was that I felt I was exercising more so I walked less outdoors. That probably contributed towards vitamin D deficiency (correctable with supplements, but you have to know to do that). Overall though it has been a good thing,
While it depends on what I'm doing, I also find that walking on the treadmill can actually contribute to my concentration.