I bought the new Harry Potter and the Half-Blood Prince on Monday morning. I won't include any spoilers here except to say that this book marks a return to form for J.K. Rowling.
Some reviewers have complained that nothing major happens in this book until the last chapter. I would say that is only the case when compared to the last two grim books. Harry Potter and the Half-Blood Prince is far more like the first three books in style and tone, and for me that makes it that much more enjoyable than the previous two dark tomes.