Former Secretary of Defense Donald Rumsfeld swears by them, the only problem is that getting a standing desk can be expensive. If you check out that wiki link to the standing desk entry on Wikipedia, you'll find an photo of Mr. Rumsfeld at his desk!
There are some health benefits, and instead of spending your hard earned cash, you could do like Josh Nunn, who re- purposed some file cabinets to serve as his standing desk. He uses two filing cabinets standing edge-on. The one with two drawers is the perfect height for your keyboard, while the three-drawer unit can hold your monitors. This can be an extremely cheap way of trying out standing desks, or hacking your own. We think that if sitting long hours in a chair gets uncomfortable for your back, trying out a standing desk like this, especially if it's a hacked one, is definitely a good idea. It's been shown that changing working position once every few hours is healthier than just sitting at the same one for the whole day. [via Lifehacker, photos by Josh Nunn used via a CC license, Mr. Rumsfeld's Standing Desk via Wikipedia, Nick Sieger via CC license and via Coming Anarchy]