Hah, I can't really speak for the Firefly crew because they were trying to speak Mandarin... but I've heard that it was horrible.
Peter's Cantonese on the other hand was passable, all things considered - which is a complement because Cantonese is much harder to mimic than Mandarin.
Oh, fun fact: "Snakehead" is what Cantonese people call the Chinese Triad/Mafia/whatever. I don't know if they did that consciously or by accident though, but I guess it's a nice coincidence.
