As a Fitness Instructor, working in a gym with 4 vibrating Powerplate machines, each costing �7K, I really wouldn't waste your time or money on them.
We've had them for 6 months and in that time, having spoken to regular users, there are no reports of any positive benefits, except perhaps helping with stretching and improving flexibility and range of movement.
If there is any benefit at all, then the best results would be seen in people who exercise regularly anyway, but use the Powerplate in conjunction with their regular routine.
Also, Powerplates and other vibrating machines have absolutely no cardio benefit so will not and cannot increase or improve fitness levels.