I've been using one for a few months now, and I find myself standing about 90% of the time. Even sitting, it's nice to have a higher-than standard surface (I'm 6'5").Mine is electrically adjustable, and the entire work surface lifts up to 48".
There are some potential problems with other types. I have set up others here in the office that are clamp-on or surface mounted, and they can have trouble with multiple monitors, for example. One worker had to give it up because the lift that was stong enough to support her three monitors was too strong for her to pull back down.(oops!)
There are others in the office are using them with a little "micro-elliptical" device like this, and I would love to try a treadmill. Unfortunately I would have to put the whole desk up on blocks to get the extra height, and that also makes it more of a "standing-only" situation.