I've been using a treadmill desk almost daily for the last 2-3 years and I absolutely love it. I had two laminectomies for herniated discs about 5 years ago and the dynamic motion of walking has helped me maintain my back health better than an ergonomic chair or a standing desk would. On days when I had a lot of meetings and couldn't walk at my desk it was noticeable.

It works particularly well for me, but your mileage may vary (literally). I tend to walk fast (2.0 mph) which means I need light clothing and a fan blowing on me. That seems to be faster than most people who walk (and beyond the speed you should start at). I'm also able to walk longer than a lot of people do (I've walked a marathon distance several times during 24 hour hackathons). Contrast this with my partner who found it almost impossible to work while walking.

I tend to pace when I think, something about walking helps me think; my treadmill allows me to stay at my computer where I can still write code or make notes, versus getting up from my desk and wandering away. This really helps me switch rapidly between deep thinking and focused working.

Two things I highly recommend if you want to take the plunge: make sure you position your keyboard and monitor(s) carefully as it makes a huge comfort difference, and use a trackpad over a mouse. I've used my Magic Trackpad and had no problems controlling my cursor, but the few times I tried to use a mouse I found it very jumpy (especially at my faster pace).

If you're unsure I recommend looking for a used TR800, take a lot of breaks, and start slow. And a quiet fan can help keep you cool.