Warning: contains spoilers.
The first few times I watched this film, I took the everyday scenes as being "real" and the apparent hallucinations as being under question (was time travel actually involved?, etc). But recently I've come to see it from an inverted perspective - the hallucination scenes where "real" from the perspective of Donnie and the everyday scenes where all metaphorical impressions that Donnie has of what is happening to him.
Some examples: the jet engine killing him in his sleep is a metaphor for schizophrenia striking him unexpectedly in his youth. Grandma Death is a metaphor for his future and Frank, the illness itself. The deaths of his girlfriend, mother and sister and the absence of his father, etc are metaphors for his separation from his loved ones due to his illness. His sister's acceptance to Harvard is related to his feelings of everyone else passing him by as his illness brings his own life to a halt. From this perspective, his laughter at the end is a much darker moment than it seems at first. He hasn't saved anyone, but he has lost himself.