Donnie Darko is one of my favourite films. Sadly, I cannot say the same for the sequel, and understand why the director of the original (Richard Kelly) refused to be involved in any way with the making of a sequel.
To explain - Donnie Darko is an extremely well thought out film, with everything seamlessly tied together at the end. And although it is also complicated, it does make complete sense to those who ponder endlessly over it. Everything is perfect. Sadly, this is not the case with S. Darko.
It seems Fisher has taken the basic elements from the original film - time travel, a complicated plot, and the future trails - and exploited them.
Let's take the future trails: he uses them more often than he should, and makes it seem on a couple of occasions that they are alive (they react to the person). Although it is fine to interpret these things differently, as a viewer, I did not think it worked.
The whole thing seems yet another film about a misunderstood, mopey teenager, which we have seen enough of in Twilight. Although Donnie in the original film was mopey, the audience were also led to believe he was slightly mad. In this film, the main character is simply dislikeable.
One final flaw - the special effects resemble something out of Smallville. They make the film so much less believeable, and prevent the viewer from becoming lost in the universe, as they were in the original.
If you are a huge Donnie Darko fan, I highly suggest you skip the sequel altogether.