I would like to avoid a dramatic fall to my death, so I'm avoiding anything that even remotely sounds like a spoiler. And yet, I still want to give an opinion on this book, so there are a few things that I feel obliged to say to the prospective reader who's wondering whether or not this book is worth reading.
Let me start off by saying that the last time a Harry Potter book was released, it was carefully thought out, and all of the books prior to it were clearly leading up to it. It has clever writing and good plot twists that J.K. Rowling built up to from the very beginning. This book doesn't feel like a natural continuation of the series, however, and it's quite evident that J.K. Rowling was only partially involved inn crafting its story.
Because The Cursed Child reads like a fanfiction. Not a horrible fanfiction, since at least it's entertaining, but a fanfiction nonetheless. If I could boil down my description of this book to one word, that would be it. What can I say? Practically everyone is out-of-character, and then "cool" things happen that feel like they happened just because a fan thought that they were cool, but otherwise feel contrived. There were also details in the story that contradicted established elements of Harry Potter lore, which resulted in plot holes. Somehow, three writers with very obvious talent got together an write a fanfiction for a sequel.
That's all I have to say about it. There's nothing wrong with liking it, if you read it and decide that it isn't that bad, since it does have a story that at the very least can be entertaining, but it just doesn't have the greatness that the original series had. Maybe Fantastic Beasts and Where to Find Them will be better, but overall it just feels like people are turning a story that was already complete in itself into an unending franchise.