As you can see from my rating, and in particular, if you have read my reviews of the other books of this series, this is not my favorite book. I think there are a number of reasons for this, but most importantly, this is the book where innocence is lost.
Before this book, even when things get dark, and Harry thinks he could die, he always holds out hope that things will be okay. That everything will work out in the end. After this book, we know that anything can happen, and that no one is safe.
Harry begins to truly move along the path that has been set for him in this book, and by the end, he is a different boy than the one we knew at the start. The world around him holds far darker things than we'd been aware of before, and the war truly begins.
This book was also the first to come out as the furor took over. And there was a huge gap between this book and the next, which I believe caused Rowling to lose the flow a bit. It's not as noticeable in Order of the Phoenix, but is quite pronounced in both Half Blood Prince and in Deathly Hallows in a way that cannot fully be explained by the darkening of Harry's world.
I think this, as the turning point of the series, is an important story, but I also think it's really mostly for the true fans. I don't think it would hold a great deal of interest for those who have not somehow been captivated by one or more of the previous books.