I watched bits of The Departed, and from what I saw, I definitely prefer the original, Infernal Affairs. Better acting, a better script, and a much better ending. But then I only watched parts. My family told me to change channels because they got annoyed at how it wasn't as good.
It's probably a great film to people who haven't seen Infernal Affairs... but I have seen it. It's my second favourite film of all time, next to Infernal Affairs 3