The first time I watched Split I was confused by the hype it but had better appreciation for it the second time. Out of the trilogy it is definitely the best but overall I'd still give it a 7.5 out of 10. I agree that the writing could have been better and James McAvoy carries the entire movie. Any time he wasn't in a scene I stopped caring. Although the depiction of DID was clearly exaggerated, no one cares because his performance was so good. They could have just made the entire cast James McAvoy and it would have been perfect.

As with most horror/thrillers, you have to lower expectations for a realistic plot and just try to enjoy the ride.