I always thought it to be a good addition to balancing the body until my partner at work had a bad experience with it. She bought a magnetic bed pad to sleep on hoping it would help with her back pain as well as anxiety. Ony after a few days she started feeling flu like and ill. She did not equate it to the pad. But as the week turned into two and so on she thought maybe it was the pad. She went to her chiropractor and told him how she was feeling, when they explored possible reasons, the pad came up. He immediately said he also had similar experiences with magnetic pads. It took her several months to feel whole again. Her chiropractoe also said it took him awhile as well to recover from it.
Others say it is great- so it seems it can be pretty powerful for some people. Depends on your sensativites.
I have used small magnets for every ache and pain for many many years. If I feel a 'twinge' in my knee, I adhesive tape a small magnet or two onto the spot .. the pain usually disappears within an hour.
For back pain I have a belt-like contraption which closes with velcro strips, and contains 12 larger magnets. It helps me a lot. I would say as much as BenGay or a pain killer. I never tried a mattress pad.
I really believe magnets help for all kinds of aches and pains.