Magnets definitely work well on preventing minerals in water sticking to pipes etc. I have used them in several houses. And if you install one in an old house , for the next few months you have chunks of scale clogging up your shower etc as they come off the pipework! I have no problem believing this, therefore.
But using a magnet to make fuel burn better seems to be utter nonsense. If there was any measurable improvement, it would have been verified by now, because it would be so easy to measure.