My friend, who is a very fit and healthy 32 year old, had a session in a gym on a power plate and said she really felt that she was being worked out, but wouldn't consider it long term (it was a free trial session) as she felt better just doing a normal gym workout.
A colleague of mine who weighed over 30stone and bought one (I'm pretty sure it was a real one, although I can't be certain) over a year ago and he is still over 30stone; I don't think he uses it often atall and said he didnt feel a difference although I could easily put that down to him not trying, as he tends to blame everyone else apart from himself for his eating habits (his weight is purely through over indulgence, he does not have a medical condition). Although that's a slightly seperate issue I thought I would add it anyway