My personal experience (and from speaking with pro trainers) is that it is a tool, like any other to be used among a variety of tools to get a balanced workout. I am not sure I would stand on that thing and do a 40 minute routine like apparently the ads suggest. Truth be told, my Grandmother had one of these in the '70s, but it had a belt attached to it...supposedly to wrap around your waist and 'melt the fat'.
And these plate machines are incredibly expensive for what they do.
What it does help is in forcing your body to use different muscle for stabilization which can be quite good since very times do we move in perfect linear movements in perfect space...its nearly always walking on uneven surfaces, lifting luggage above our head or carrying lopsided audio equipment down the stairs!
Try doing pushups slowly on the floor for 45 seconds...then try it with hands or feet on the plate when turned on for 45 seconds...harder on the plate...I am told because it is also the slightly different muscles being called in to counteract the vibration...which don't always get as much of a workout. So in that regard, a good thing. But again, only in the context of a more balanced workout involving free weights, cardio-oriented workouts, etc.
Just my two cents.