Note: there may be some spoilers!
As much as I love Robert Downey, Jr as an actor, in no way does he physically fit any description of Holmes anywhere. Much like Hugh Jackman being a full foot taller than his character, Wolverine, RDJ does not possess the height, physiognomy nor even the eye color of Doyle's detective. Nor did Jude Law's Watson ring entirely true: he had the limp, the moustache and the good looks of Watson, but was taller than Holmes and more slender than out to have been.
As for character, the only ones that could be considered true to the original were Watson, the soon-to-be- Mrs. Watson, Mrs. Hudson, Lestrade and Professor Moriarty. Holmes' character was nothing more than the sum of all the known and surmised eccentricities and slovenly about his person; Irene Adler started off strong, but descended into damsel-in-distess/at-the-mercy-of-powerful-men type of rubbishy character. The villain, Lord Blackwood, was not as venal as he could have been, but his wardrobe was fantastic.
London is portrayed in all its unglamorous, crowded, filthy, horse-dropping-bedecked street beauty, all in murky browns and greys; 221B is far larger and jam-packed with objets, dark, smoky and somewhat repugnant.
There was a feeling that the director did not know what to do with the female characters- while Watson's fiancee had the strength of character her Canonical counterpart had, she was filmed oddly and mysteriously, for no apparent reason. Irene Adler was filmed mostly in headshots, her cheekbones and pouty, mauve-hued lips first and foremost.
This movie was incredibly fun as an action/comedy- if viewed as a non-Holmesian piece, it is thoroughly enjoyable. The only impediment to that is having substantial knowledge of the original material- lines are sprinkled here and there from the Canon only serve to highlight the considerable departure from it.
There is plenty of action, as can be expected in a Ritchie film. The relationship between Holmes and Watson is well drawn out, but not overburdened; the Holmes/Adler relationship is dutifully sketched - kindred souls with an unfulfilled potential, and some unnecessarily sexy bits tossed in for....well, for no reason at all. The story, filled with the occult and political machinations, moves along and all is revealed, in true Doyle style, at the end.
Rating: 4 crowns out of 5