Spellbinding is a good description because you don't know what's going to happen next but your intuition whispers to you softly, "something bad". The character James McAvoy plays is criminally insane from years of abuse at the hands of his mother which adds to 'the pathos" of the story although when "the Beast" appears, pity becomes a foregone conclusion. A monster created a monster. I won't watch this movie again. "Go sell crazy somewhere else, we're all stocked up here." Yes. I borrowed that from "As Good As It Gets", a la Jack Nicholson.
"Signs", another M. Night Shyamalan creation, I do not mind watching again even though I know what's going to happen. "The Sixth Sense" is a good movie though I pass on it when I have the opportunity to watch it. Not certain why. Perhaps because it sneaks up on you. It teases you regarding the finality of death. The mother with Munchausen's Syndrome by Proxy who slowly poisons her daughter to death. You have no idea Bruce Willis' character is dead until the last 5-10 minutes of the movie. These are sad stories within a sad story and I don't do sad stories.
"Split" certainly keeps you in suspense. The fact that the one personality James McAvoy plays, "the Beast", takes two point blank shotgun blasts to his upper torso and lives, let alone walks or breaths or talks, etc.? Uhh, no, that requires way much of my imagination to imagine anyone being able to do that. So. Perhaps we will see what stops the Beast in the next installment(s) of Shyamalan's stories. Maybe. Maybe not.