I read this book back in August when I was visiting my sister. She had put her name on hold at the library for a copy of this book; and when it came in, she was part-way through a book and told me that I could read it first.
So, what was my opinion? Knowing that it was written by J. K. Rowling, it seemed to be very much in her style. What I remember from the Harry Potter books was her extreme overuse of adjectives and adverbs. Nothing could just happen or just be; and that held true in this book. The aforementioned sister remembers that in the Harry Potter books, the verb of most sentences was "said". That trait (fortunately) did not carry over to this book!
This book is a mystery. I have to confess that I like my mysteries fairly fast-paced, and I found that this book dragged. Coming in at just over 400 pages, it took me several days to get through it. And I did guess the resolution to the mystery before the end, but don't worry - I'm not going to post it on here!
So I guess that my overall opinion would be "meh." I probably won't be actively seeking out any of her other books, though if the opportunity arises (like it did for this book), I would probably read them.