Might have been superlative but for the ending I should mention at the start that I saw the Director's Cut, not the original. Also, I don't want to go through my thoughts on the whole thing since other people have written far more capably on this movie than I have.
Overall, I thought the film was filled with excellent acting and ideas, especially Gyllenhaal, Barrymore and McDonnell. However, the ending wrecked it for me.
In essence, Donnie Darko gives you two choices: Donnie is either schizophrenic, or he is actually involved in a wild sci-fi adventure to save the universe.
Since Donnie is already, as far as we can tell, seeing a psychiatrist / therapist and on "medication" prior to the first appearance of Frank the rabbit, it is reasonably clear that Donnie *does* in fact have genuine mental problems (otherwise, why is he seeing the psychiatrist? unless the rabbit has been preparing things for a while?). We are later told, however, that Donnie has been taking placebos. We are (and, again, I assume) therefore supposed to assume Donnie is *not* schizophrenic, even though the film's strength would be its portrayal of a schizophrenic's world.
If Donnie is not schizophrenic, then the second choice presents itself: everything we are seeing is in fact real, and Donnie is genuinely following the tenets of a book written by a woman about time travel / multiple universes. But the "reality" approach is also a problem: the tenets of the book read like pure hokum - they seem two dimensional, childish almost - yet they are supposed to be grounded in science. Assuming that we are supposed to believe the world is real, the time travel book looks like it was written specifically to justify the plot of the movie. No explanation is given, for example, of why it is necessary to have "manipulated dead", or quite why this relates to Hawking's theories on space time which are referenced earlier on. Actually, I felt that the book worked better as a product of Donnie's schizophrenic imagination, since it sounded so obviously ludicrous that only Donnie himself could have been expected to believe it, and it is simply not tenable that Donnie's physics teacher would actually refer Donnie to both Hawking and a book that sounds rather like a children's story of which he happens to have a copy in his briefcase.
So, Donnie is either schizophrenic or really following the book. At the end, Donnie is back at the events that took place at the beginning of the film. Instead of escaping the fall of the aircraft engine, which led to the original course of events, he chooses to lie in his bed, the engine falls, and he dies. And with him dies the film.
If Donnie is schizophrenic, then the effect of the ending is that the entire episode was a dream, because he dies before the events happen - i.e. it never happened, so has zero emotional impact. This is the same problem that, incidentally and in my opinion, ruined David Lynch's Mulholland Drive.
If Donnie is *not* schizophrenic, then we are expected to believe that Donnie has successfully time travelled / whatever back to a point in time when he had a choice. On *that* interpretation, there *would* be an emotional impact to Donnie's decision. *But* the principles by which he has proceeded are so patently ridiculous that the film loses all believability and becomes the worst kind of science fiction, where the rules can be made and changed at the will of the author (much in the way that Tolkien managed to save Frodo and Sam at the end of Return of the King by flying in eagles without any prior explanation, destroying the otherwise convincing sense of three books' worth of writing in one fell eagle-swoop). All the hints in the film that Donnie is a Messianic figure, choosing his sacrifice, come to nothing because the rules Donnie follows are not credible.
In essence, to me at least, the movie was a choice between a fantastic look at mental illness with the most cop-out ending possible or simply a pathetic excuse for a science fiction movie. Either way, the ending ruins it.
Which is a crying shame.