The power of this film derives from its ambiguity

Rating: 8/10 (8 December 2009)

I agree with other user comments about how the Director's Cut spoils the film by removing the ambiguities over its meaning; instead the viewer is asked to accept a load of sci-fi twaddle about time-travelling.
What really comes through for me in this movie is the disturbing effect that mental illness, and the drugs used to treat it, can have on a person's sense of self. From personal experience I am very aware how fragile reality, and particularly the linear nature of time, can appear to those in the grip of such a disturbance.
By extension, it is impossible to say that such a view of reality is not at least plausible; and I would also point any film lover to the movie "Jacob's Ladder", which also explores the idea of alternate, parallel realities.
"Jacob's Ladder" spoiler coming ... !
To say that Jacob Singer, the protagonist in that film, is simply experiencing a hallucination destroys the power of "Jacob's Ladder" in the same way that making a similar assumption about Donnie Darko does. The point is that both explanations may be true, and may exist side-by-side.
The final scene of "Donnie Darko", where Donnie lies on his bed laughing as he experiences the epiphany of having all the story's events fall into place in his mind, is truly frightening - but it is so because of the ambiguous nature of those events; not because he's some time-travelling superhero, but because he is mentally ill and his view of reality is corrupted. Likewise, the film is affecting precisely because when it ended, you're not sure what happened.
It is this quality that makes the film so powerful, and the Director's Cut with all its silly exposition about time travel just makes it into a geekfest. A triumph for editors Sam Bauer and Eric Strand, then. (Yep, had to look them up.)