Maybe it's a bit like Peter Brooks little crystal box.
But I do have to say it DID make a difference to my previous car. I've got some dyno figures done before and after, if I can find them. Off the top of my head it was about 3 or 4kw. Can't prove the fuel savings except to say that I felt I was getting better fuel economy after it was fitted.
BTW, here's the website and a pic of it (it's the small one on the right).