from what i was told, the main problem is with gas engines. the hho messes witht the o2 sensers or something, and forces the comp to put more gas in than is needed. you can fix it but on gas you cant just hook it up and go, and you get less of a boost. on diesel however you get much more of a gas savings.
they guy i originaly got this idea from ( he has one on his truck) his son sells a set up for big trucks in alaska. they went from 8mpg to almost 14mpg ( so he says), the trucks virtualy never shut off so theres no problem with the containers freezing.