This movie starts off really well, with the introduction of one of those old and festering upper class English families, where everybody hates each other, and their suppressed frustration and rage boil over into bizarre and hurtful behaviour. And it sets up what should have been a nice dichotomy throughout the movie - these horrid and spite-filled shells of people, as compared to Dr. Watson's good natured befuddlement - it would have been fun to see these character's usual tactics of subtle terribleness totally fail to work on Watson, because he was too stupid to understand what was going on, and too nice to assume they were being mean to him.
But the movie looses this thread after about ten minutes, when the most spiteful and hate-filled of the family is murdered, and the characters all stop being interesting. That's not to say that the movie itself becomes bad or anything, it is still entertaining in its own ridiculous sort of a way. But the start of the film was so promising that I was kind of disappointed.
I was also kind of annoyed by the clock that struck thirteen times before anyone was murdered - there was no attempt to give this a scientific or rationalist explanation. They never claimed that the murderer had arranged the clock to strike thirteen times to fill everyone with fear, or anything, they just sort of... have it there. It seems to be implying the existence of actual magic, and this is difficult for me to take in a Sherlock Holmes film.