Finished it Saturday morning. Unfortunely, I caught a glimpse of the huge spoiler and knew before it happened. I would have definitely prefered to keep it a secret, but things happen and there is nothing you can do about it.
Onto the book;
1. Huge setup for the next book, even moreso than GoF. I just hope it doesn't take three years to get it.
2. Every one of the main characters (Harry, Dumbledore, Snape, Sirius, etc.) has a flaw that comes back to haunt them big time.
3. The book doesn't follow the previous four's plot wise - bad summer, going back to Hogwarts, something magical happening, studies and Quidditich, huge revelation, plot wrapped up nicely. In fact, this book does a 180 - still a bad summer, but that's because of the magical act happening which results in Harry possibly getting expelled. No Quiddtich for Harry as he's kicked off the team, the revelation does happen but only after Dumbledore wises up (as explained very well in the book) and the plot is left hanging sort of. Just look at the final chapter title for a hint.
All in all, not as good as PoA (my fav) or GoF, but a worthy addition to the series.
