I built one (from Ikea parts, inspired by a posting on ikeahackers) very recently.

I use it far too short a time (2 weeks now) to say whether it really helps, but it's definitely a change that makes me happy _so far_.

I feel more awake (no 'slumping'). I move a lot, walk in circles and just move away from the machine, when I feel I need to think about something.

Will I keep using it from now on? Only time will tell.. It _is_ definitely less comfortable than reclining in a good chair.. :)