I haven't tried magnetic BB's, but I've tried the hock and stable boots. They do make a bit of a difference. I'm planning on getting Spighi a set of magnetic stable boots when I can get the money together, as he tends to get wind galls and friends have seen a drastic reduction in the size of galls on their own horses, through the use of magnetic boots.
Yeah I think they're worth a shot.
There is no scientific proof, but they do seem to loosen horses up. The magnetic rugs and quarters sheets are meant to be quite good too for horses that are tight over the back.
I do plan to try the back on track stuff too... I did order the magnetic boots.... after I ordered them I read everyone's replies and guess what... Back on track has boots too! I'll look at getting some of them later on...