There is no good evidence for any health benefits from magnetic products available to the consumer. If you are thinking of buying a magnet to strap onto your arm or leg or wherever, you are investing money in an unproven product.
However, there is promise in the use of pulsed magnetic fields from specialized electromagnets in the treatment of depression. This therapy involves transcranial magnetic stimulation to stimulate nerve cells in the region of the brain associated with depression.
In 2008 the FDA cleared a device called Neurostar for people with depression who have not responded to medication. The results of the treatment are mixed but preliminary reports seem to indicate that it has some value.