The final Harry Potter book (and it is final, there's no coming back from this one) is much better than it's previous two episodes. It's one of the best written of the bunch, even if the epilogue seems patchy and tacked on.
Rowling got back to doing what she does best - simple plots with interesting character interaction. She also makes you understand, and even like, some of the more repulsive characters in HP lore - an impressive feat.
It is also going to be impossible to make it into a movie without it being at least 3hrs long. Or should I say... a good movie. Order Of The Phoenix was messy and chopped up so much it looked like it had gone through a blender... it stretches the limited of what they could do in a short movie.
Then again... half my friends list seems to be reading it, so you'll all find out.
3 & 4 were my favourites, but 7 is definately up there along side them, even if it isn't a complete story on its own (which has been the bigest problem with the series after 4).