The first 15 chapters cover Harry getting to school, starting his NEWT classes, being the Gryffindor quidditch team captain and trying to find what Malfoy is up to. It is well written with a good mix of dialogue, action scenes and substories.
I think the substory of the various romances between various characters slows down the book.
What is a real suprise is that Book 6 justifies and underlines many of the things discussed in Harry Potter and the Chamber of Secrets. This was a book that I wasn't too impressed with, but thought was a good movie.
Book 6, so far, is a good read with a steady pace. I can't wait to read the next 15 chapters.