I've been using a standing desk for 9 months now. I'm a software architect and my days run 8-12 hours, depending on what fires I'm putting out. I found an Ikea Jerker desk on Craigslist for $75, then picked up an industrial anti-fatigue mat at Home Depot for $20. It only took me about 2 weeks to realize that I can't go back to sitting all day.

You're not exercising like a treadmill desk, but you will often find yourself moving about as you work. Heck, some days the music kicks in and you've just got to move with the beat while you're building a new feature for your app.

Side effect: after about three months, I was also able to give up soda completely and have almost completely given up caffeine.