I got my father this model of the chi machine years ago and he really liked it. He even told one of my aunts to get one for her health.
If you read through their website, they list the reason for why they maintain that steady speed and states that machines having variable speeds can actually do harm. Not sure if it is true or not, but I did not want to mess with my father's health to save a few hundred dollars. Would be interesting to know if owners of the variable-speed machines get the same benefits listed or if they experienced any negative reactions.