Well I don't believe they live up to the "lose all your body fat overnight" claims! But they are like anything else in a gym, a tool which you can use as you want and get results from depending on how you use.
I think it got rid of my tendonitis. I used to have what felt like an anterior shin splint. It KILLED every time I tried to run and got worse so that I could feel it throbbing when I took rest days. I did try to rest it but I have a slight....gym obsession and couldn't not exercise, so I tried to reduce what I did, a year later still the same pain and if anything worse.
I did stretch but I don't think I ever really stretched enough, maybe 60 seconds per pose max. I used the powerplate to do my stretches and it really did help. I now after about 24 months of pain so bad it was rare I didn't have a slight limp, am able to run and run and run, no twinges no sudden shooting pain and no afterward throbbing. I could say it was a coincidence but knowing all the things I had previously tried, I don't think so.
I think the powerplate works as it vibrates to such an extent you can stretch that bit further, a bit like dynamic stretching but alot more intense. Also as it has a timer your aware of taking a short cut and reducing time with each stretch you hold. I now notice I can stretch further, both hands palm down flat on the floor besides my feet, legs straight when before I may have been able to just touch my toes on a good day. Took me a few weeks to get to that stage on the powerplate.
I think the idea makes sense, the clenching and unclenching of your muscles which in part helps to build muscles, especially core muscles, it would also boost your cirulation- warms you up, good for cardio as you start of a sort of buzz and don't get that building of momentum thing going on, its already there.
Don't knock it till you've tried it!