I've been using a standup desk for a year now. It sucked a for a while. My legs would be really sore after only a couple hours, and I would kneel on my chair to take the weight off my feet. Since then things have stabilizing. Before I made my desk extension I read a lot about using one. Most of the articles I've seen detailing others experiences are overwhelmingly positive about their experiences. I disagree with that sentiment. For me its a mixed bag.
Obviously there are some benefits, otherwise I wouldn't keep going. Firstly, my legs are stronger. Standing, walking, and running are less tiring. Now I can enjoy hiking without focusing on my endurance, and not be completely miserable while running. I don't think I've lost any weight, I'll probably have to fix my diet for that. My posture is way better. My neck and back feel stronger and fitter. So yeah plenty of physical benefits.
There are some work related benefits as well. High energy activities are quite enjoyable while standing. I can stand up, rock out and bounce around. Programming, working with others, and writing are awesome. I've also experimented with standing up after hours playing videos games, this is also pretty fun.
Physically, I'm not tired at night, but my joints ache quite a bit. At the end of the day I'm tired of standing. Also, If I've hurt myself playing ball, haven't had enough sleep, or have a hangover, standing blows. When it's really bad, I'll take down my standing desk for the day. Taking the desk down is a pain in the ass, so I try to avoid it.
Work wise, I have a lot of trouble going email, documentation, or reading in general. The only exception is code. I still like reading code. Repetative tasks are also more frustrating when standing. However, that might actually be a benefit, since I'm motivated to find improvements.
One thing that I don't do standing up, is planning, and diagramming. I prefer to do my initial diagrams, plans and equations by hand, and I'll site down for that.
So like why keep going?
I've experienced positive and negative effects after a year of standup desk. There's good and bad both physically and mentally. So why keep going? The balance is tipped by my internal desire to have a more active lifestyle. I'm in love with programming. And programming is one of the most sedentary activities imaginable. A stand up desk is one of the few ways I can be more active while doing what I love most.