These devices have been reported to work in some situations, but not in others.
The underlying theory involves altering the growth of calcium carbonate scale with an ordinary magnetic field. The magnetic field changes which form of calcium carbonate crystal (calcite, aragonite, and vaterite) is favored. Certain forms of crystal make tough scale while others can pass through a water system without scaling.
The growth and precipitation of carbonate crystals are affected by many other things such as calcium concentration, carbon dioxide concentration, temperature, pH, and the presence of other metallic ions. Exposure time to the magnetic field, magnetic field strength, and magnet orientation also have an effect.
The different forms of carbonate crystals can be tested to have specific properties under controlled laboratory conditions. Under uncontrolled conditions, the scientific literature is full of mixed results and mixed explanations. Because so many things affect the formation of scale, it is impossible to say if a magnet or device generating a magnetic field will work to thwart scaling in any specific situation. It may work wonderfully in some situations but not at all in others.
The best way to tell if such a device is worthwhile would be to see if several people using your same water supply report it as having worked lately. If water management practices change (e. g. factors affecting the formation of scale are controlled) then such a device may cease to work. If no one using your same supply can attest to its operation then this would be a risky purchase.