About it not really being a Holmes story, from what I've read about Holmes, it seems the film/trailer is hitting on some accurate details:
Holmes does carry a pistol and Watson was shot at one time
Holmes did prize fight
Holmes was a cocaine user
Holmes used martial arts and killed a man
Holmes used disguises (not sure about this one, but I'm sure it will be there)
Holmes was a ladies man
Holmes didn't believe in personal hygiene and Watson was a neat freak
Ritchie has also said that the Doyle stories have action and the films haven't really explored that. Also, the deerstalker isn't actually in the books, that was from some illustrations of Holmes by artists. So when Downey shows up in a fedora, that's also a bit accurate.
So it seems to me the movie might be right on.