Having just gotten back from "It", I can safely say that it is excellent, not just as a horror movie, but as an adaptation of the book. It captures the themes, characters and pants-shitting terror of the only book to have ever spooked the bollocks off me. It's an absolute blast, and if you don't want spoilers, I'd stop reading here.
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Okay, if you're sure!
The time-shift to the 80s works in the film's favour, they put a lot of effort into it without ramming it down your throat, so that it doesn't just feel like a "Stranger Things" parody (despite the appearance of Finn Wolfhard, more on him in a bit) and actually gives characters room to breathe and evolve. As an added bonus, when they are adults, it means that they won't be fighting the creature with zimmer-frames, knitted jumpers and copies of Glen Miller tapes.
They focus on the first part of the story, the kids, and adapt the tale in their own way whilst still honouring and adding touches to the source material. Case in point: the street names are exactly the same as in the book, there's a Lego turtle by Georgie's bed, the statue of Paul Bunyan is seen, some pieces of dialogue are verbatim from the novel, and there's even a blink-and-you'll-miss-it cameo by Tim Curry's version of that fucking clown.
The chief rule of horror movies is usually not to show the villain/monster, but here it works BECAUSE they circumvent that rule. Some of the known ones are there (Georgie chasing his boat, Eddie encountering the Leper) but some are changed for the better or equal effect: Stan's encounter with the statue is removed, thank Christ, and replaced with one of the most horrifying pieces of artwork I have ever seen, in a sequence which makes me want to burn the concept of paintings themselves, and all buildings just to be safe; Mike's backstory is edited, mostly for the better and to remove the references to "The Shining", and instead of the bird in the pipe (one of my favourite pieces from the novel of batshit insanity) we get something... darker but nonetheless effective.
The characters are done incredibly well. Each of the seven kids are sketched out fully and played by genuinely good child actors, which is the thing which would have made or broken this film. The standout, as I thought he would be, is Richie Tozier, my second favourite character from the novel, who allows you to laugh and rest easy after sequences of existential dread and wiping the urine off your seat. Ben is still prevalent, but sidelined after a while in favour of Bill and Beverly, which streamlines it somewhat, and makes it no longer "The Adventures of 3 Kids Beginning with B, and Some Other Assholes" as was in the book.
Even Stan, the character who in the book gave me psychotic lapses into existential dread brought about by boredom, is given stuff to do here, including that aforementioned fucking painting. However, Mike is still somewhat shortchanged, making it still "The Adventures of 6 Kids... And A Black One, I Guess", though it's not that jarring and makes sense. Still, I like Mike.
The child orgy is also removed, as is the Patrick Hockstetter sequence involving a fridge full of leeches, though Hockstetter himself is still here (presumably to make his cameo in "Firestarter" later...). One of those things is for the better, and I'll leave you to decide which.
In summary: A great story, a great adaptation, spooky and atmospheric throughout, terrifying in parts, embracing its source material with love and affection without being pandering; and making enough changes to keep it fresh and excellent.
Ben is still the best character.
PS The photograph sequence, the best, most frightening part of the trailer, is just as effective in the movie, and a lot longer, actually rising in tension.
