These things are weird. I've actually used one in a triathlon shop I helped out in and much to my disbelief - I assumed they were most likely a nonsense - it made a difference to my ability to balance and move against resistence.
However, the power of states of mind over the body is a well established thing so it could still easily be a nonsense. That, however, works both ways. And, somewhat in that sense, I'd be just as wary of claims of scientific 'proof' against it as I would of the band itself.
Edit: I was just thinking, in a pragmatic sense, it doesn't matter if they don't have the physical effect, via the route claimed, but rather they tell a story which somebody can get into and, as a consequence, perform better through a kind of belief. Anyway, whatever, I shall not be buying one.