It

This movie is quite possibly the most beautifully shot, strongly acted, and entertaining films in the fantasy horror genre since, well, Nightmare on Elm Street III. I know that's a highwater mark to use, but this feels like a spiritual successor to everything that movie set up.
So, points to touch on Direction/Cinematography/Music: Beautiful. Andrés Muschietti and Chung-hoon Chung combine together to create a vision of Derry Maine that feels like everything that every King book has tried to convey about Americana, but ultimately fails until now. This feels like the town, and it feels like the 80s. But it doesn't particularly scream it's time and place to point where it becomes distracting. What can be distraction however is the score, which while beautiful, can be a little overpowering at times.
Acting: The Loser's Club is amazingly well acted, with not too much focus on anyone character. Bill and Beverly take center stage however, being the best actors, and with a script that serves them the most. But the film leaves a lot of room for everyone else to shine, with perhaps Mike being the weakest, but only again because everything else outshines the reduced role he's given. Richie, Stan, Ben, and Eddie are wonderful comedians in their own right, with Motor Mouth Richie getting a lot of well earned yuks in, but Eddie's neurotic babbling can easily steal the scene at times.
Plot: This is what adaptations should strive for. The beats that needed by there for the film to be truly called "IT: Chapter One" are there. And that's what may partially keep this at a 9.5 for me. The core of the book's initial exposition is here, but it diverges at interesting points. Rather than being Universal Horror icons, IT uses core fears against the kids, in combination with specters of past events.
On that note, this movie is full to bursting with references to the book that are shown, not told. The water tower, the Paul Bunyon statue, the Canal Street, and many other references to locations and elements of the book that the film just doesn't have time to deal with.
The film makes a major departure from the book after Nebolt Street, in choosing to follow the three part Hollywood Act structure. The Loser's Club briefly disperse as they are split between Bill and Richie. Bill wants to return to finish Pennywise off, but Richie tries to flee, leading to a scuffle, and a falling out.
This may be a sticking point for a few people, as the film slows down for about 5-10 minutes, but kicks into high gear as Beverly fights off her father's rape attempt, only to then be snatched up by Pennywise as bait for the boys. It might be another sticking point to make her a damsel, but it actually flows back into place, and acts as a way to introduce us to the true nature of Pennywise with his Deadlights.
No, there is no spider, but we get a glimpse into the fact that Pennywise is a truly primal creature with layers that this film isn't willingly showing us. He's playing his game by ear, and ultimately is horrified in the climax to find that all his shapeshifting can't scare the kids, who corner him. He makes what looks like final parting words as he 'dissolves' away into a well that goes even deeper into the sewer. We of course know there's more to come.
Pennywise: Skarsgard chews scenery with all 10,000 of his teeth, to great effect. I'll take a moment to give this film the .5 ding that I can't ignore: The CGI does glare at times. There's no getting around it, you know when you're seeing a CGI effect. But the imagery they go for in the transformations is so uniquely affecting that I applaud the attempt. The true form under the clown makeup is the stuff of Lovecraftian nightmares, and IT does not hold back from beating the crap out of these kids, while gleefully mocking them the whole way. There are sequences in the scares that just could not, I repeat, COULD NOT be done without CGI, and they do try to use practical whenever possible. The blood sink scene is a sight to behold, for example.
Similarly, his other forms are...interesting. There are three other largely special effect driven creatures: The Lepper, The Headless Boy, and the Flute Woman. The creatures have a sort of Tim Burtony plasticky quality, that somehow manages to work. They have a fairy-tailish quality that fits with the fact that these are fears drawn from the children's imagination, and thus are more grotesque and luridly malformed, rather than being grizzly and covered in blood.
That aside, the movie does fall into familiar traps: Jittery camera movements, a few false scares, things that the Hollywood engine refuses to avoid using.
All in all, this is a really fucking strong movie with a LOT of heart, a lot of laughs, and a good number of both creeping dread moments and ambitious jump scares that try to do a bit more than your average ghost movie.
Other interesting notes *For those interested about the character of Patrick Hocksteder, he is not quite the sociopath that King described him to be. Instead, he gives off something of a pedophile (As much as a 15 year old can give off) and sadist coding. It's interesting that his death is at the hands of zombie children.
*There are few fears in the finale that heavily, heavily hint at where part II is going. Unexpectedly the Mummy and a few other forms are pulled out of Pennywise's bag of tricks in his final brawl with the kids. There's a lot of skips in time, especially when the group disbanded, where Part II can go in and fill in some blanks with further childhood encounters with IT and it's many forms. Hopefully they get a start, so that the actor don't age too far out of their parts.
*There is no stinger at the end, which I'm kinda glad for. For those wondering, they intend to have a finished script by January, and will move promptly into production once that's drafted. As I said earlier, it feels like they've already written and shot a lot of material that leaves openings for things like the history of Derry and why it's inhabitants are so strange to be investigated in Chapter II.

I think that Georgie's LEGO construction in his room that gets dropped is meant to be a reference to It's enemy Maturin.

One of the most effective forms being the twisted flute-playing woman from the picture. Additionally, when It manifests from the projector screen, I was temporarily thrown off by it looking like a special effect, but then I immediately realized that it was absolutely what It would've looked like in that form. I also like that you only really get a glimpse of It's spider-like limbs once.

(Also, my only minor complaint would be that the fight with it is a little bit shaky and quick-cutting, but I feel that it's overall forgiveable)

I thought it was really interesting how Bev's dad literally couldn't see the blood that was all over, and it makes It's influence on the older townsfolk more apparent (like the older couple who ignore Ben getting attacked). It makes me REALLY curios about Eddie's mom though, since she seems to be aware that something terrifying is out there and is trying to actively protect him from it was the Placebos/Gazebos that he's always taking.

I loved Flute Woman. I feel like she was an interesting attempt at portraying the fear an OCD person has.

• In the opening, how it cuts and shows that it's their mom playing the creepy background piano music. SO smoothly executed.
• I love that everything that IT writes with its own blood, it's always flowing upwards. I think the first time you see it is in the mini doll coffin in the wellhouse.
• Additionally, the TV show that IT controls in the background that's always playing when the parents are around, but they just don't pay any attention to it whatsoever was a really good way of showing IT's influence over the town in ways that the kids are often actively aware of, and the adults never respond to.

Good points about Mrs. K. I've never read the book, but that makes a lot of sense as a background. I figure that we'll start to get a better understanding of just how thoroughly IT really controls the town in the sequel since we'll have a lot better opportunity to show the things that they all have to struggle with as adults.

It'll also be interesting to see how much, if at all, they utilize the child actors, since in that interview they mentioned that a few of them have already grown a bunch, and their voices have changed even just over the course of filming, let alone between the gap that a sequel would take place over.