Okay, so I've seen both the Director's Cut and the Theatrical versions of Donnie Darko, directed by Richard Kelly. And no, not the sequel by Chris Fisher.
Anyways, both versions of the film are creepy, dark, and utterly twisted. The main difference between the two is that the Director's Cut gives you a little more information about what's going on.
Really, I'm having trouble doing this without giving away spoilers. And I'm trying to be unbiased, but honestly, it's a very well-written, excellently directed, and awesomely spooky film. Both versions.
So the bottom line is this: if you like to figure things out, and be thrilled by bizarre semi-psychological-possibly-really-happening surreal weirdness, watch the Theatrical. If you like to know a bit more about what the heck is going on, but still want the surreal weirdness, watch the Director's Cut.
If you don't like surreal weirdness that explores dark alternative realities, then maybe you shouldn't watch this at all.