i fancied them when they first came out but was put off by reviewers saying they felt dizzy for hours afterwards. I am dizzy at the best of times, so dont want to risk it!
there is usually a list of heatth conditions posted beside these machines which if you have you shouldn't use the power plate. the list includes diabetes & cancer, which would suggest that there is potential harm - on the other hand it could be just to stop people suing the company
i have used PPs on and off for a while - they are very good, particularly for massage after a gym session. like with most things, moderate usage is probably harmless