It's this simple: HBP is where Rowling starts going off the rails. It's a DREADFUL book, and I have no intention of seeing any film derived from it, nor Deathly Hallows (a truely TROLLish mess of a book).
figures

HBP is a great book due to the history of Voldie.
Rowling could have done ONE exposition dump...er..."special lesson with Dumbledoore"...and given us that and spent the rest of the time having Harry actually taught something USEFUL for fighting Voldie and the DEs.
And don't get me started on the "romances"....Ginny (aka the "Potions Princess"), who has been a cipher the previous five books and all of a sudden Harry has the raging jealous monster reaction to seeing her with another? Remember how Molly said the "had to help" Arthur along?
And Ron/Hermione? GAHHHHHHKKK! (aka the "puke smilie"). NO basis. Ron is an ASS (and proves it in the next book for sure, as if it weren't clear enough). But Hermione is all over him like a rash? The SAME Hermione that almost told Harry she loved him WAY back in book one?
Rowling is a good "world builder", and her writing style is very evocative. But her plot logic is paper thin at best, and her sense of character dynamics is utter crap. HBP is the first book where Rowling's pluses no longer total more than her minuses.