Beyond the chorus of "you have to see this" and the knowledge that it's supposed to have a mind-blowing twist, I knew nothing about Donnie Darko when I started on it. Not even who was in it, and so I was surprised to see how many people I recognized; I had gotten a vague idea it was a low-budget movie somehow. I also don't remember if I saw the theatrical edition or the director's cut, as it was months between when I loaded it onto my Archos and when I watched it.
All in all, I'd say I enjoyed it, and it held my suspense as I tried to figure it out. By twenty minutes in, I'd gotten a grasp on the tone and style, and after that, it mostly went the way I expected; yet there were some surprises, notably on which of the various plot threads were picked up and which never panned out.
Much of the credit the movie's gotten is in how you really have to think to figure it out, and even if you think you have, you can't be entirely sure. I certainly appreciate that you have to think it through, but what I am not decided about is if there is an answer. The appeal of a mystery loses a lot (not all, but a lot) of its edge when it becomes clear to me that either there isn't an answer, or if there is, the filmmaker deliberately didn't give us enough information to figure it out, no matter how clever or deliberate we think about it. That's just too easy a path to the appearance of profundity. As Nietzsche said, "Mystical explanations are considered deep. The truth is that they are not even superficial."
For an example of what I mean, compare Memento with Primer. The former includes everything you need to figure out what was actually going on, but boy, is it quite a puzzle to actually figure it all out. The latter deliberately excludes a lot of necessary clues, and deliberately obfuscates much of what it does include. I enjoyed both movies, but I respect Memento a lot more, and I consider the "it's such a puzzle!" aspect of Primer to be a red herring; it's not really a virtue at all, it only seems that way at first glance. I'm not sure where Donnie Darko falls on this spectrum. It's certainly nowhere near as bad as Primer, and I feel it quite likely that the filmmaker did have actual answers in mind, but I suspect there's some things that are simply unexplained in the film. That said, I admit I might just not have put it all together. I didn't work as hard on the mystery as I might have, mostly just because I didn't have the free time to do so.
One thing it has in common with Primer is that I don't think the filmmakers really intended the mystery to be the point of the movie. The primary point of the movie was the feel. The mystery isn't nothing, and it's not merely a device for establishing feel, but when the two conflict, the tone wins. So not answering some questions, and not picking up on some apparently storyline elements, is an intentional way of directing the feel of the film even when that interferes with the idea of treating it as a puzzle.
All in all, I appreciated the movie, and of the many movies I watched during my recent travels it was one of the more suspenseful and enjoyable, but it's not one I would hurry to watch again. There's nothing wrong with the movie. It's well produced, well acted, and tightly scripted. It just doesn't grab me. The feel and tone sustain my interest only so far, and the mystery doesn't make me feel compelled to rewatch it to try to solve it.