The idea is unusual and perfectly realized. It's killer, unexpected. But far from just a bloody mess, as many people expected. There is some drama and depth. The plot is non-trivial. With a point. Not just entertaining, but for a deeper viewer.
The acting is top notch. Vince Vaughn played so convincingly that it was believable. There was no doubt about his performance. Some moments were touching, some were laughable. Catherine Newton is exceptionally good. She, too, without any doubts believed. She is both admired and enjoyed her performance. She did a great job reincarnating.
About some of the plot points. First, I'd like to say something about the theme of friendship raised in the film. The theme is explored in great detail and taste. It was just great. Maybe this movie teaches how to appreciate friendship, as well as to be a good friend(s). Second is the theme of love. Rather teenage, but still pure, sincere and real perhaps. The theme is also well covered, without much drama, but very interesting. It was a funny, original, interesting love story. I say love. Not just a crush. Because I would have preferred this kind of love to that of Romeo and Juliet or some other 'adult' story. There's a real, strong and sincere feeling behind the unserious shell.
The idea behind the film is bombastic. The well-known 'switched bodies' story has been transferred to the genre of comedy horror. Talent was enough not to spoil the idea and implement it properly.
There is not much to say. Personally, I really enjoyed watching it. Though I can't stand violence and cruelty. But apart from the 'sea of blood' there's much that is 'mine': funny, unordinary, beautiful, touching, dynamic and deep. I'll definitely watch it again.