The structure from the novel, where the adults don't remember events, is frustrating in this - in the book the adults and kids cross-cut, so the adults remembering would naturally spoil the kids bits. But here, we've seen everything, and so much of the movie (especially that choppy choppy choppy first hour) is made up of "I don't remember ... wait, I kinda remember..." bits.
Ryne is right about the distracting CG used to de-age the kids - Finn Wolfhard has a CGI sheen through the whole movie. It hits Eddie and Ben the most after that, but the effect is at its worst in their first full new scene.