Hahah, that Honest Trailer nailed something that kept bugging me throughout the whole movie. Can I at least get a glimpse at the other fifteen personalities? Grant Morrison wrote a character called Crazy Jane with this exact same schtick for his version of Doom Patrol (in fact, I'm pretty sure this is where Shyamalan got the inspiration - The Beast being the equivalent of Black Annis) and part of the fun was how a previously unheard of personality would pop up for a specific scene or moment.
Shyamalan's bizarre career arc is so unique. He peaked with Unbreakable and has since acted the cinematic equivalent of a chastised husband stuck in a vicious relationship who hasn't stopped apologizing for The Happening when he proceeds to unfold The Last Airbender on his audience. So it makes sense that, for a film which is a return to form, he'd do an Unbreakable connection. I don't get people who say that this ruined the story for them. It's not as if the whole film is a "secret sequel". It's just one little coda at the end.
Anyway, McAvoy is as excellent as everyone says. The story is really risky and compelling, cleverly combining Horror movie mumbo-jumbo with a more developed and modern approach to mental illness. Let's hope for less After Earth and more of this in Shyamalandingdong's future.