I have been sitting for years and it was starting to show. I was skeptical about a treadmill desk at first, wondering if I could really type and walk with accuracy, but after just a few hours (at a really low speed) I began to get the hang of it. I had to raise the desk a little higher than I expected but once I found a good height (just above my elbows) I was fine.
My job as a medical coder allows me to work from home 2 days a week so I wasn't concerned with the size of the desk fitting in my office although I have told everyone at work about it and I've convinced one of my co-workers to order one (I hope the two of us can convince management to buy some for the office).
There is one weird side effect I found after a few weeks - it is more difficult to stay sitting for long times. When i am in the office, I get restless in meetings. At my desk I have to stand up and move around more. It's almost like my legs are telling me to "get moving". Either I have to find a way to get my treadmill desk in the office or work from home more days because I love my treadmill desk!