The story is interesting to begin with, with a bunch of kids creating an "end of the world" story and then watching it made come true when they are adults. They embark on a mission to save the world, as their original story prophesied.
However, the plot is really weird. There is a virus that kills people by making their blood explode, but it seems it is not contagious, it needs to be sprayed on. A giant robot smashes through Tokyo and the police stop in front of it to shoot guns. The leader of a hypnotic cult becomes the leader of the most important party in Japan, and so on and so on.
Then, as world saviors go, the heroes are the most inept I have ever seen. The movie is very long, at 142 minutes, and it is only the first part of a three film story. The acting is pretty bad, too.
Bottom line: it seems this is a failed attempt to mix the success of the Death Note franchise (I don't know what that thing was successful either) and the comic book/manga craze that is taking over the movie world in the US and the rest of the world. There is a feel reminiscent of Watchmen in the film, but only a trace. I am not saying that it is not worth watching, but I can't say I liked it.