I find it a lot easier, these days, than sitting. I can move around, I can pace a little. One of my clients only has sit-down desks and, in addition to being really stiff in the knees afterwards, I always feel cramped and a little restricted. I notice it, and it distracts me from what I'm working on.

My standing desk is also just high enough to fit my exercise bike under it (if I take off the handlebars), and that is really nice.