A nice thing about seeing movie posters ahead of time (apart from the obvious advantages of them telling you when the movie will come out, who is in it etc.) is that they often give you hints as to the tone the filmmakers will be aiming for in the finished feature. Judging from the new poster that has just come out for Harry Potter and the Half Blood Prince they're going for very dark and very moody.
I personally found the last two Harry Potter books to be far too long and extremely tedious so I don't have high hopes for the last two movies. I was extremely confused at the news that they would be splitting the final book into two movies since I felt there was barely enough interesting material in the book to fill a trailer. But that's just me.