I got one after my doctor recommended it. He was jazzed about the Trek Desk, which is a small-ish folding desk that fits over most treadmills. It seems like a TV infomercial product, but it did what it said, and it was a cheap way to start and not have to buy a super heavy integrated treadmill desk.

I loved walking while I worked. I can't stand and work, my feet start to hurt surprisingly fast, but walking I can do all day. I felt better, and it cured my afternoon post-lunch crash, I no longer went through an hour of trying to stay awake. It helped my back feel better too. One hard part about using it for me is the constant stream of interruptions I have, people needing to talk to me during the day. I have to turn it on and off a lot. (The Trek Desk, btw, made it slightly harder to reach the start/stop buttons.)

Even with them being small, one of the biggest problems I had is the setup is still bulky. The treadmill takes a lot room in a small office. It's pretty noisy too, even just walking, so consider the noise if you're in a shared work space.

Linus Torvalds seems to use a treadmill with his standing desk: https://www.youtube.com/watch?v=fuS-3HSnpq4

I did but the one I bought was too narrow so I've put the idea on hold for the time being.

I know someone who used to play warcraft this way.