I have done these in the past. I think the basic premise is the hot water helps open up pores in your feet while the ionic metal pulls out toxins through these pores. You will see alot of different colors in the water afterwards but I have no idea if these are toxins. The skeptics claim you can run the footbath without your feet in it and get the exact same colors.
I cant seem to find any literature verifying these actually work though. It seems a study where they examine the contents of the water for heavy metals would be simple enough to do but I dont see any studies supporting these claims.