I've been using a standing desk for years now, and while you're not exactly doing jumping jacks the whole time, I certainly move a lot more than I do when I sit down. Just shifting weight from leg to leg, changing standing position, occasionally stretching/touching my toes. You end up making many more small movements than when you're just plopped in a chair.

I found the adjustable stand up desk to be really only part of the ideal solution. Standing all the time aggravated circulation problems, so I added a treadmill specifically designed to be used at a desk. Now it's easy to walk miles a day, at whatever pace fits with the task at hand. The desk is on wheels, so I roll it away from the treadmill when I want to sit. I vary between sitting, perched sitting, standing, walking, and reclining while on the computer. (Working at home.)

I also just ordered a Cubii under-desk elliptical, for use at the office where a treadmill and moving around would be intrusive. Like many here, my job and several hobby projects involve working on computers, and all that sedentary time adds up if you don't combat it.

I think a standing/adjustable desk is a good solution, but you should be able to sit down and, more importantly, put your feet above your waist for a while, occasionally.

Otherwise you may ruin the veins in your legs in the long run.

(I'm not a doctor.)

It's the other way: standing is necessary but not sufficient. Exercise doesn't cancel out sitting and blood pooling in your legs.

Any comparison with these big sitting rubber balls we see from time to time ?