Really amazing -- I happened to re-finish Book 1 the evening before started reading Deathly Hallows. I was stunned: the whole story of Book 7 is set up in Book 1.
Rowling has said that the whole story just sort of unfolded for her over the space of a few days, and it's taken her this long to write it down. I believe her.
I think, in spite of other's comments about it not being the "best written" or "classic literature" that is one of the amazing things about the whole series.