This is one of the best vocaloid songs and it could have made a fantastic manga, but this just wasn't developed well. The art is gorgeous, but there's so many loose ends in story and character.
What in the world is going on with Kaito's backstory? I don't even know where to begin asking the questions. His father stabbed him as a child and he forgot about this up until now? It adds nothing really to the story and I just didn't get this.
The love triangle is very, very complex, and everyone seems to have a different story and a different take on what's going on. It makes for interesting storytelling, but three unreliable narrators in a book that's only a little over a hundred pages can get tedious and difficult to understand after a while.
The ending is extremely ambiguous and not really much of an ending. It would have made more sense for Miku and Kaito to have died, and Luka to have killed herself afterwards. The ending is a very unclear cliffhanger that just didn't make sense in the context of the story. We still don't know who Kaito chooses.
Miku slowly going mad was a really cool thing to see though, and I always enjoy a dark, twisted one-shot. I just wish they had spent a little more time on this and done more editing so it would make more sense.