I find my feet get sore if I stand still for long, but not when I walk. A speed around 2 km/h works well for me.
It is necessary to have the monitor and keyboard at proper heights, but that is also true of sitting desks. Bad and badly placed monitors and keyboards, and laptops in particular, are an ergonomic disaster either way.
I suffer from chronic lower back pain due to a herniated disc, and never has my back felt better since my injury in high school. Sitting is absolutely terrible for it, and standing, while an improvement, leads to poor posture because I found I would lean on a particular leg for too long, and muscles in my back would get unevenly strained.
I walk between 1 and 2 MPH all day long. I find it very easy to do all computer tasks while moving. The only time I stop is when I need to write something down on paper, which I can't even do well at the lowest setting of 0.4 MPH.
It is a bit tiring, but in a very good way. My two "breaks" are sitting down with my team for lunch and then sitting on the couch for a little while after I get home. I did not work my way up slowly as some people recommend; I had been using a standing desk for about a year and I just dove right into the walking because I prefer it to standing even when I'm a little tired.
A big part of the value of the treadmill for me is that it forces me to always move and bend my legs, which means I never have the opportunity to default to a bad posture. That's the reason why I walk all day every day.
A cheaper alternative I considered was getting a Bongo Board: http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3D...
I went with a treadmill desk because I thought with the bongo board I could still default into a balanced inanimate position, and because I'm fortunate enough where I could afford a treadmill desk without it being a huge sacrifice for me. But, if that weren't the case, I think the bongo board could be an improvement over standing.
I want to believe there's a way around this: like a monitor that follows your eyes... I guess it'd have to be worn as goggles
I rest my elbows on the pad on the front, with my keyboard about a foot forward on the desk. No matter my body's movement, my arms are stable and anchored.
With respect to eyesight, there is mild movement but I don't find it bothersome or difficult. First, it is very mild - maybe an inch or two between steps. Second, it's more of a front-back movement than up-down (remember, this is a walking treadmill, not a running treadmill), so I find it does not at affect my field of vision nor what my eyes are focused on.