Biometrics are good for usernames but not passwords. It's much harder for someone to get something out of your head than cutting off your thumb. - and as playeren says, it's impossible to change it.

http://news.bbc.co.uk/1/hi/world/asia-pacific/4396831.stm

Don't use biometrics for anything but convenience type features. You can't change your fingerprint if bad actors gets a hold of it.