I've since seen Harry Potter and the Half-Blood Prince.
I think this film highlights one situation where it might be impossible to film a book. It's to do with the way the books get longer, darker and slightly more adult through the series, while the plots get more complex and interwoven. At the same time the fan base still includes a) a lot of 11 year olds and b) a lot of people who know the books inside out, both of whom must be satisfied at all costs.
HP1 (known to UK residents as the Philospher's Stone) set a precedent by being very faithful to the first book and more or less keeping everything in. For the sequels it would have been harder to cram everything in - so directors have had to choose what to cut. Alfonse Cuaron did this creatively in HP3 (the Prisoner of Azkaban) deliberately telling a much simpler yet still coherent story.
By the time we get to HP6 the director is faced with an impossible choice - keep it all in and your film is too long or too cluttered; cut it all out and it won't make much sense. David Yates has taken the second option. He's kept the humour and much of the adolescent drama, and sown the seeds for the real tragedy of the piece (that Harry ends up dating Ginny Weasley not Luna Lovegood) but cutting out so much else, including the Severus Snape and James Potter backstory, that the whole meaning of the Half-Blood Prince title is lost. Unless you know the book you will be baffled by the ending.
What's to be done? In my opinion there's nothing wrong with the production of this film. As with all the Potter films, casting is perfect. I could single out anyone but here Jim Broadbent (Slughorn) and Evanna Lynch (Luna Lovegood) steal every shot they're in, while Tom Felton (Draco) and A..l..a..n.....R..i..c..k..m..a..n (Snape) are more intense in each film. The effects are superb as usual - and while I doubt there's a single non-effect shot in there, much of the work is nicely understated or atmospheric and so doesn't detract from the drama. Despite the confusion the film is extremely funny and occasionally exciting or sinister.
David Yates is pencilled in for HP7 too - and there's the possibility of splitting it into two films. If this is the reason for it then I approve. It's one solution to the dilemma of HP6, and I will look forward to the next installment.