Personally, I quite enjoyed Harry Potter and the Half Blood Prince. I found that all the characters were three-dimensional, and the script was engaging and entertaining.
I understand that many parts of the movie were pruned out, but if they had stayed in, the movie would have been 4 hours long! Perhaps WB should have split this in two also?
Overall, I have no complaints at all on HP6. I believe that the screenwriter did an excellent job filling holes in the script, and the special effects were astounding.