Review (4/5): Fun idea, but difficult in practice
If you really want the looks pictured in the photos, be prepared to spend a ton of time. It seems like such a cool thing, but in reality holding a magnet over each nail for minutes at a time is difficult and uncomfortable, and then if you add a top-coat, basically needs to be re-done because the particles settle again.
That said, the colors are cool. Even applied flat without a magnet, you can get some interesting looks.