I've always been told that chipped/badly worn enamel shouldn't be used as it contains some pretty poisonous stuff.
I've also been told that the groovy, orange, 70s enamel contains
really poisonous stuff.
I assume it's not an issue if the outside's chipped, but I'd be careful about it if it's damaged where food will be in contact for long periods.
My main issue with enamel pots (not the sexy Le Creuset style
cast iron/enamel combo) is they are just too lightweight for me.
I suppose they'd be fine for boiling and so on, but I'm a burner, and can't imagine enamel handling the regular scorching/scraping my stainless pots get!