I've never used one, but my understanding is that they are plastic-coated metal, correct? Pretty hard? Flavored, or not?
I've had good luck with rubber or plastic composite bits--with no metal interior--in the past. If you have a chewy horse, he'll gnaw through it over time, and with hard plastic, that can create sharp edges. The key is to make sure that the mouthpiece isn't too thick, because usually it is considerably thicker than a metal bit's. I don't think that HM bits are any "gentler" than metals bits, but rubber ones are due to their give and flex. But if your horse for some reason hates metal, I suppose it's worth experimenting. Some people swear by them.