I have heard of them and I know at least three people who have tried to build them. I have never seen one work and I know of nobody who says they have seen one work in person.

edit - also, I went and had a bit more of a look and it turns out there is an energy difference between magnetised and demagnetised materials, but it is very small. One thing is that spontaneous magnetization happens and it wouldn't occur if the energy difference was large.