I just read it on holidays and have to say that it was an excellent book
Some people don't like it because it's not like the other Potter books but the books have been getting more mature as they go along and this is a natural progression.
One of the things that I like is that Rowling mixed it up a bit. In the other books you could pick out the surprise or the bad guy early on. Some of them were good surprises some of them not so good (come on, how many people really thought that Sirius was going to turn out to really be a bad guy when you were reading PA?) but the new book really kept me guessing. I thought that this book was going to end up being about werewolves. They introduced the werewolf leader early on and I thought that the surprise was that Draco and Tonks had been biten by the werewolf and that Harry was going to confront the werewolf leader at the end. I was also convinced that Snape was going to turn out to be a good guy and that he was going to be the one to die.
Rowling is setting the series up for life after Hogwarts but I think that Harry will somehow end up back at school in the next book. My guess is that during the summer he finds out that one of the horcruxes is at Hogwarts (maybe Gryffendor's sword or the Sorting hat) and he has to go back to school to find it.
Overall I give it a 9/10