I converted my desk to a standing desk this morning (couldn't sleep and there was nothing better to do). Already I can feel the difference to my posture. I do understand what some people say when they complain about the pain in their feet but I think the benefits outweigh the drawbacks. You expend more energy, meaning it's one extra step in preventing obesity. Your posture doesn't slouch. Finally, since you aren't sitting, you can't really relax and do nothing. Standing desks force you to be doing something, otherwise you're just standing there.
Humans evolved a long way to stand on two feet, there's no reason why we shouldn't take advantage of that.