Even if they did work, they wouldn't be pulling the heavy metals out of the CNS.
Maybe they are beneficial, I really don't know. But I would think that something like sunbathing or saunas or hot baths would be equally or much more effective via sweating.
I'd like to find a good study on them. Not just something that they've removed metals, but whether the amount of metals they removed was significant.
Some people have felt better after using them. But I don't know if that has anything to do with metals, or perhaps they detox in other ways.