Roughly two weeks ago I bought fifteen dollars in cinder blocks so that I could set up my standing desk. I'm fortunate in that my office has room for both a standing and a sitting workstation. The arrangement served well with regard to trying out a standing and a sitting position when working.
The first week was awful. Standing was uncomfortable and I found myself generally unable to focus, but I knew that was going to likely be the case. I forced myself to persist even if it meant doing something mindless like reorganizing my workflow or backing up files. I made sure I spent at least five hours a day standing at the desk and working toward one project or another.
My work, tedious or otherwise, continued as normal.
After acclimating to the standing position I found that I had probably set the desk two inches too low relative to my height. This made using my tablet to draw difficult. It was relocated to my sitting workstation. For everything else it was as good or better than my sitting workstation.
Your stance will vary depending on the setup of your standing desk. I do recommend standing away from the desk at least ten inches or more depending on the length of your arms. Avoid positions that encourage leaning on the desk as this will increase the time it takes for you to get comfortable standing. This allowed me to lean in an relieve pressure on my wrists without getting too close to my monitor.
If you find yourself really needing to lean in on your desk, get a footrest instead.
Health benefits of the standing desk? In dealing with the portions of my workflow I find boring (but necessary), I have much better stamina for them at my standing desk. If you find yourself getting sleepy at your workstation and using inordinate amounts of coffee or other stimulants to stay awake, a standing desk might be for you. Aside from the impact on your health such things have, there's the time and energy used preparing a cup of hot bean juice.
Being able to more quickly and efficiently deal with the banal things I find myself doing at my desks helps me get to the good stuff I like more quickly. Relative to my mental health, that's definitely a benefit.
I love playing computer games at my standing desk. I find that I'm more alert and able to play more effectively than if I were sitting. Also, I tend to be able to play longer, if I desire, before growing tired or restless. These days I can only play a video game for about an hour before I get jumpy and desire to go and do something else. The standing desk makes an evening of WoW or Spiral Knights with my friends more tolerable and enjoyable.
All told, for generating raw text for my works, nothing beats my bean bag chair, a semi-dark room and my MacBook Air or iPad. It's really nice to have options though.