I think it can make a huge difference - but only if you are doing it right! The idea is to do pilates-type exercises on the machine whilst it vibrates, giving you a better workout in a shorter time. I find that it is great for targetting specific muscles that are difficult to get at otherwise, and as the workout only takes 15 minutes 3 times a week, you are more likely to do it than, say, an hour long Pilates class 3 times a week.
As an example, when I had a c-section, they cut the muscle between navel and pubic bone that I didn't know I had, but that made it difficult to sit up in bed or in the bath for a few weeks. Once healed, I forgot about this particular muscle until my first powerplate session, and boy did I remember then! The next day, I had problems getting out of the bath! I'd done lots of stuff on the powerplate for stomach and waist, and the next day I felt like I had abs of steel, muscles down my side that I didn't know existed and a really toned stomach .... but that I was wearing a great big fatsuit over the aforementioned abs of steel etc.
It is great for increasing muscle mass, toning and circulation, but there was an piece in the Telegraph yesterday saying it shifted fat too. Will try and find it.
I would recommend it, but make sure you get lots of training on it.