Went to see Harry Potter and the Half Blood Prince in Melbourne last week with a group that included midnight_jen and partly_bouncy.
midnight_jen, the big Potter fan of the group, was disappointed because so many of her favourite scenes were omitted from the movie. partly_bouncy, no Potter fan, called it "forgettable". Personally, I thought it was fairly good. Obviously, there are compromises that have to be made when turning a book into a film and this was too large a book to film completely, so some tough choices had to be made. Not all of these choices were made wisely. In particular, the already long movie rumbles on beyond its logical ending (with everyone poignantly raising their wands in tribute) to splutter out through a dialogue with no discernible purpose other than to establish some stuff that will be covered in that movie anyway. The background of Tom Riddle (which should have been in Chamber of Secrets in my opinion) is skipped, but this disconnects the main storyline from the stuff about snogging.
One thing I noticed was a great deal of British slang in the movie, including "snog" and "knickers". They'll probably overdub this in the US release.
I do have a favourite scene. At one point Hermione points out to Harry that girls are only interested in him because they think he's the chosen one. Harry beams, and says "I am the chosen one!" only to have Hermione brain him for being a prat.