Your opinion on Mary Sues?

I personally can't stand them, as soon as a Mary Sue appears in a book, fanfic, movie etc. I have to stop reading/watching. Tron Legacy was nearly ruined for me when that damn Mary Sue (Coora? Kora? Forget how to spell her name xD) appeared....don't flame me for that, she was a Mary Sue! But regardless, what're your opinions on them?