Saw it last night.
Personally, I thought it was fantastic. I had relatively high expectations, and it exceded them.
It was very true to the "kids" part of the book (minus the 50s Universal Movie Monsters and the kiddie orgy).
They made the loudmouth kid afraid of clowns instead of the movie monsters, which seemed kinda redundant. Obviously the original "kids part" took place in the 50s, and this in the 80s (which I think is necessary for an adaptation of the book 30 years later), so I read they were going to switch out the Wolfman/Frankenstein/Dracula/The Mummy with Freddy/Jason/Pinhead/Chucky, but ultimately decided that would have been too campy and cheesy. I think it might've worked though. Georgie's ghost, the gushing blood & hair, the dismembered Easter Egg hunt kid corpses, and the leper (although, they could have laid off the CGI on that one) are still there though. They added the lady in the painting, which I guess was pretty good if you're gonna change shit up.
The biggest knock I heard on the movie was that it wasn't scary. IT was more a story ABOUT fear than scary in and of itself. I wouldn't expect the movie to be scary (and what movies truly are terrifying to an adult anyway?), and actually if it were just wall-to-wall cheap jump scares, I would have been disappointed. I was actually pleasantly surprised at how creepy Pennywise was. That projector scene and some of the scenes in the abandoned house were damn freaky.
The one positive I heard was how well acted it was by the kids. I had heard the girl and the kid from Stranger Things were the most remarkable, but the asthmatic nervous kid with the overbearing mom I thought was by far the most impressive. He's got a career ahead of him.
I thought it was great and look forward to the second half.