I use a standing desk a lot (six of eight hours, doing it for the last couple of years), am a bit unfit and pudgy, and I'm doing fine. I can get in the "zone" just the same as sitting, and as far as I can tell it hasn't negatively affected the quality of my work.
To me, it feel like it kills the post-lunch haze, and reduces the friction to move around. I find it easier when brainstorming to pace or just walk away to a whiteboard, for example. I shift my weight from one foot to the other, bend my knees, and generally move around a bit during the day. I take sit-down breaks every hour like you'd take a stand-up break.
I tried it out by stacking some things on my desk[1]. Make sure to keep the same sort of ergonomic setup with standing as sitting (eg. monitor at eye-height, arms flat to the keyboard, etc); staring down at a laptop is just going to make you unhappy.
I'd be careful before jumping on the stand-all-day bandwagon, though: http://ergo.human.cornell.edu/CUESitStand.html
(And beware of confirmation bias[2] when first switching over. If you're happy with it at first, give it a little while before deciding whether or not to go for the long haul. Last thing you want to do is regret dropping the cash on a standing desk.)
1. http://instagram.com/p/MUmlP/ http://instagram.com/p/SWS7gDO6Tg/ http://instagram.com/p/Jy1d6Qu6cj/
2. http://en.wikipedia.org/wiki/Confirmation_bias