Donnie Darko is (to me) one of those movies ruined by the director's cut. Not only due to the music changes, but also because the "message" of the movie is made more explicit and on the nose. And reading Kelly's interviews and opinions is even worse. One of those movies where keeping the original, and the mystery, is absolutely mandatory.
I haven't seen Darko's director's cut, but am reluctant to do so just because they swapped out Echo and the Bunnymen for the opening scene, which works so well.
For Blade Runner director's cut, I had mixed feelings. The unicorn scenes and some other added footage helped. I thought getting rid of Deckard's inner thoughts VO was a mistake.
In the scene at the noodle stall, it clarified a lot of things such as the language people were speaking. The VO after the dancer's shooting was also revealing in a way that interpreting the expression on his face was not.