It is common knowledge that you shouldn't judge a book by its cover. However I do not think you should judge it by its author either.
I simply just adored this book. When I first began reading it I was forced to finish; I had absolutely no choice in the matter. It was mesmerizing and not at all what I had expected.
JKR demonstrates a breath-taking ability to illustrate and describe her characters, from every possible angle. No character in this book is truly good nor truly bad; they're just humans, blinded by prejudice and personal flaws. The psychological value in this book was utterly fascinating to me.
I enjoyed following the various plots, and I loved the ending where everything joined together in a complete whole. So many different perspectives and aspects of the same story gave me a lot to think about.
This is by no means a "feel-good-read", but it is an elegant and relevant book about politics, ethics and people. Consequently I think JKR has proven herself capable of writing to adults as well as children and teenagers. It may not be what I expected; but one should never underestimate the value of a good surprise.