the master key lets you skip a couple annoying areas and a couple bosses but I don't tell people that until after experiencing blighttown.
Don't want to devalue the joy by taking away the suffering before it happens : p
I am now convienced that the master key has hurt more players than it has helped.
Watching someone trying to figure out how to get into new londo because they always used the master key and they didn't ever know about the chest with the key to new londo just made me face palm hard.
No one should be allowed to use it on their first playthrough, it should be unlocked after NG+.