I wore a magnetic knee straps for months and didnt notice any benefit, so it now sits in the cupboard redundant. It cost about £20.
I also had a copper bracelet but it was uncomfortable and I didnt notice any benefit with that either, so it also sits in the cupboard!

I know many people who swear by the copper bracelets.
Ive never heard any positive or negative feed back about the magnetic stuff.
Heather ;