I loved the first installment of It (which you can read about here) and even back then was immediately eager for part 2. I'm also really glad that they split this into two movies - usually I'd roll my eyes at Hollywood milking a franchise, but It is a vast brick of a novel and I think a big part of why the mini-series failed is because it's too convoluted, switching back and forth between the main characters as kids and adults and telling their stories simultaneously. It works in a book that's split up by chapters and headings, and by splitting the film into two halves it's the best way of getting that across without confusing the plot.
Overall both movies are really accurate to the novel - there are deviations from the plot, and the ending of Chapter 2 was completely re-written (with constant jabs at King throughout the script for not being able to write a decent ending, with even King himself getting in on it with a cameo) but there's not a lot from the book that I really missed as most of the key elements were there, and those that weren't I could tell why they were omitted.
The biggest thing that I did really miss was I felt the movie could of done a better job of delving in to the insidious influence that Pennywise holds over Derry. In the novel there are several instances throughout the history of the town where really awful, terrible actions have been carried out by the residents, and it's all because of Pennywise feeding off of people's fear and hatred. The two main events caused by Pennywise are The Black Spot, a club frequented by African Americans that becomes the site of a racist massacre, and the homophobic murder of Adrian Mellon. The Black Spot is briefly glossed over in the first movie, easy to miss entirely if you're not looking for it. Chapter 2 opens with the murder of Adrian Mellon which plays out exactly as it does in the novel, but unlike the novel there's never any reason for the scene being there and it just comes across as gratuitous violence. Derry is supposed to represent everything wrong and hateful in American society, a haunted town with Pennywise orchestrating and feeding off of every negative event. And the movie completely misses the mark on that aspect, and simply tells a coming of age story with Pennywise targeting a few specific kids - their own personal nightmare, rather than society's.
Chapter 2 also wasn't as scary as the first movie. Although I love horror, I'm also a big weenie and deliberately saw an early showing at the cinema so that I wouldn't have to go to bed straight after and feel scared, but for this movie I just wasn't scared at all. The worst scene for me was with Beverly in her old home, but I found most of the scares were ruined with really obvious CGI, which was a shame as I remember thinking how seamless the CGI was in the first movie. I appreciate that CGI is hugely necessary in a movie like this and don't mind it being used generally, but it was at the point where it interfered with Bill Skarsgård's performance and made him feel less threatening. And my final moan is with Harry Bowers, a plot line that just didn't really go anywhere or seem necessary and I wish he'd been used to better effect.
Overall I really loved it though! I think they did a great job of adapting it to the screen, which is a bigger task than most give credit for as adaptations that stay too close to the novel usually suck as it's a different medium, but at the same time you've got to keep the original fans happy. The casting was phenomenal, the adult cast really did look like the kids grown up and at no point did I wonder who was supposed to be who. I really loved their chemistry and the dynamic between the older and younger versions, everyone was perfect and personally I thought the humour was great. Richie and Eddie were my favourite, and I loved the angle given to their relationship in the closing scenes.
I can't wait until it's released on DVD and I can watch the two movies back to back as I think that's how they'll play best. Chapter 2 probably doesn't stand up as a movie in it's own right, like you couldn't just see that one and understand what's going on, but I also feel like the clue is in the name with it being called 'Chapter 2' and there's no reason for it be a stand alone movie. I only mention it as I know that's been a bit of an overall complaint, but I don't see how it's a criticism myself.