There is no consistent independent testing that supports the claim. There are many studies that show that magnets and magnetic fields have no effect and the few that show a reduction in scale build-up are often flawed or biased or have specific industrial applications. For the average home owner I think you should spend your money on a traditional water softener with a brine tank. The dissolved sodium chloride is minor. The excerpt below is from this article here.
Consumer Reports magazine (Denver 1996) tested a $535 magnetic water treatment device from Descal-A-Matic Corporation. Two electric water heaters were installed in the home of one of the Consumer Reports staffers. The hard water (200 ppm) entering one of the heaters was first passed through the magnetic treatment device. The second water heater received untreated water. The water heaters were cut open after more than two years and after more than 10,000 gallons of water were heated by each heater. The tanks were found to contain the same quantity and texture of scale. Consumer Reports concluded that the Descal-A-Matic unit was ineffective.