It's far from perfect... there's a lot of "So, anyway, you were saying?" lines that serve little purpose besides breaking up long expository monologues. And, much like Revenge Of The Sith, there is a bit of forced political timeliness in the climax of Dumbledore and Harry's private lessons... though Rowling handles hers far better than Lucas did his. Yuck.
I think it's one of the better ones of the series. I've always been partial to The Prisoner Of Azkaban as a favorite, since it's the first book in the series that has an ambition to be more than just a children's story, but this may actually be Rowling's best work. I'd rank them as follows at this point:
- Prisoner of Azkaban
- Half-Blood Prince
- Goblet of Fire
- Order of the Phoenix
- Chamber of Secrets
- Sorcerer/Philosopher's Stone
I don't think there's any shame in Sorcerer's Stone being the worst of the series... by no means is it bad, or even merely mediocre. (Certainly not as bad as its entirely unimaginative film adaptation; thank God for the adaptation of Azkaban.) The recent entries have just been that much better.