Are people who praise this film, simply playing a prank?
God I hated this movie. The writing was terrible, especially all the dialogue spoken by the old shrink woman. All her lines came out of nowhere, and didn't have anything to do with whatever McAvoy was saying. All her scenes could easily have been cut out, as well as all the flashback scenes from the girl's childhood, to make it into a more claustrophobic experience where the whole film is in the basement, akin to 10 Cloverfield Lane.
And speaking of McAvoy... was his acting really that good? It felt like a better actor could have done much more with each identity. Take a movie like Cronenberg's Dead Ringers, where Jeremy Irons plays two brothers, and gives both of them so much character that the illusion is perfect. You don't even have to hear Irons speak before you recognize instantly which brother is which, even though they wear very similar clothes. McAvoy though... half the time I couldn't tell which character he was supposed to be, and as I was watching it with subtitles for hearing impaired, I was often surprised when the subs said that it was Dennis or Bary speaking, even though it was impossible to distinguish them.
And last but not least: The beast! What a disappointment! The whole film keeps building up the anticipation, with the shrink woman talking at length about how people can change their physicality, and we expect some great creature which will come and wreck havoc.. And what happens when he finally arrives? McAvoy takes of his shirt. And THAT WAS ALL! Even the gory scenes where he was supposed to eat the other girls, were TERRIBLE! There's a short scene where the black girl was lying on her back with her guts torn out, and it was just a tiny bit of minced meat on her stomach. They could have gone all out and made it so that he was really ravaging them, but no. Just a few tiny scratches, and that was that.