Do you mean the magnets that hook onto the fuel line?
I was rather gung ho about this a while ago and tried it out with some very powerful hard drive magnets and saw no improvement. Of course I am only one person, but I dun think they do anything, and the science doesn't really seem to be there anyway. Easy enough to try though without spending any/much money.
I personally think they don't work, as I've yet to see any data that confirms the "magnet" theory. As Tony points out on his website, people have been claiming that magnets are miracle workers for years now. I once knew a guy who sold pillows and blankets with magnets in them claiming they would help with back and neck pain. I think that the "fuel saving" aspect of magnets is just the next claim in a long line of claims for magnets.