It's too bad that kneeling chairs are often unmentioned in these discussions - they offer the main benefit of the standing desk (a natural/healthy position for the spine) with much less cost (< $100), set-up and foot/joint strain.

You can see a variety of these with a simple Google image search. Your legs just rest comfortably underneath your body, so you don't strain your back trying to counterbalance your leg (the main problem with chairs). I have a cheapo office chair and a $75 kneeling chair that I switch between (basically just when I feel any twinge of discomfort). After getting used to this setup, I don't really have any back pain.