The newest Harry Potter book is really a play written by Jack Thorne based on a story by Rowling. The change in format affects the storytelling -- a lot of the pleasures of Rowling's world-building and descriptions are gone, but the dialogue shifts the focus from plot to relationships. It takes a little getting used to, but quickly becomes a compelling (and quick) read. A bigger issue is the fact that unlike the original seven, this story is clearly meant to appeal to adults, and those skeptical of fantasy in particular. Underlying the play was the sense that Thorne was telling us "yeah, it's got wands and spells, but it's really about fathers and sons!" It's not that that's a bad thing, necessarily, but I missed the whimsy and fun of the originals.
There were enjoyable parts -- I got a kick out of how many people in different timelines were shocked by the idea of Ron and Hermione's marriage (that pairing always seemed forced), and I liked that the plot had a time-turner as a MacGuffin. Rowling created them for Prisoner of Azkaban, but quickly realized their existence pretty much ruined the plot of the series, so she had them all destroyed in Order of the Phoenix. With this story, Rowling and Thorne explain just how dangerous it is to mess with the timeline; a hoary subject, to be sure, but necessary to show how they should not have been used to try to defeat Voldemort.