I've used a treadmill desk (LifeSpan) for a few years off-and-on and I love it. Personally, I do not like standing at a tall desk. The feeling of "just standing around" never quite goes away. I slouch, fidget, shift my weight from one foot to the other, and even look around as though I'm stuck waiting in line for something. For some reason the act of walking (very, very slowly mind you) doesn't bother me at all, and I feel engaged with my work the entire time. I have not done any testing to gauge my productivity compared with sitting down in a chair for hours. Even if I learned there was no performance improvement while walking, I still prefer to walk than sit. Depending on your priorities, the most obvious difference will be aesthetic, it does wonders for your...rear view.
A more portable, less expensive option worth looking into is a "balance board," and unstable platform the side of a skateboard that keeps you physically engaged by tilting in a surfing-like stance.