There are alot of fake Powerplates around, which to me look as though they could cause damage to the joints.
The gym I work at has had a few genuine Powerplates installed but I'm still pretty sceptical. They probably work better in conjunction with a regular weights and cardio routine, not instead of. Also, you gain absolutely no cardio benefit from the machine.
The massaging mode is relaxing though and using the machine for stretching does seem to improve flexibility.