There is no health hazard. They are made from "konyaku" potatoes. Shirataki is shaped like a noodle, but not really used like other typical noodles. It is just put in hot pots, like iammatt suggested.
Konyaku is popular as a diet food in Japan though, so I guess Shirataki would be good as well. Konyaku has similar flavor and texture as shirtaki, but is more like a whole block (like tofu), rather than shaped like a noodle.