Sherlock Holmes and Watson are engaged in a mad chase up the river Thames. Their quarry is on a boat ahead of them, and as they pass by the many bridges of London, Holmes screams down to the men belowdecks to put more coal in the fire, as they must catch up. Just as Holmes’ boat is about to get the bad guy, a tug blunders between them and they have an exciting near miss. Finally, Holmes’ boat pulls up behind the bad guy boat and he and Watson raise their pistols. When the bad guy raises his hands, they shoot him. After he falls they realize he had just shot a blowgun’s poisoned dart at them, and it barely missed, passing just between the two friends.

This isn’t a scene from the Guy Ritchie Sherlock Holmes but rather The Sign of Four, Sir Arthur Conan Doyle’s second Holmes novel, published in 1890.
There’s a steady stream of Sherlock Holmes haters out there, and while I didn’t find the movie transcendent by any means (I gave it a 7 out of 10), the complaints about the film’s perceived lack of faithfulness to Doyle’s creation kind of get on my nerves, especially when they come from folks who haven’t seen the movie or folks who are obviously not familiar with the original stories.
Of course, being familiar with the original stories isn’t necessary; I have read only a handful of Holmes stories myself. But coming out of Sherlock Holmes I found myself fascinated by what did and didn’t come from the canon, so I began doing some research, and I found the film to be surprisingly accurate and faithful. 
First of all, it’s worth noting that Doyle himself didn’t take the Holmes canon all that seriously. He wrote this things to make money, and his relationship with his creation was cantankerous enough that he killed Holmes off just to quit writing stories about him (Holmes, of course, came back to life when the fans complained enough. Every weird aspect of modern fandom – the continuity hounding, the fans making demands of the creators, fanfic, all started with Sherlock Holmes). The Holmes stories can be hard to reconcile, as Doyle never bothered keeping track of the details. For instance, Holmes is initially introduced as a guy who only studies stuff that would impact his criminal investigations, but as the stories went on he was retconned into being a connoisseur of the arts as well as versed in ancient languages, none of which would be of much use when tracking a murderer. In A Study in Scarlet Holmes goes so far as to say that he did not know the Earth revolved around the sun, as it had no bearing on solving a case. 
The Sign of Four is one of a number of times when Holmes uses a pistol. In that same novel Holmes’ bona fides as bare knuckle boxer. He introduces himself to a prize fighter like this: “The amateur who fought three rounds with you at Alison’s rooms on the night of your benefit four years back.” McMurdo responds by saying, “Ah, you’re one that has wasted your gifts, you have! You might have aimed high, if you had joined the fancy.” (thanks to Wikipedia for pulling that out)
Watson also makes mention of Holmes’ talent with a sword, although he never uses one in the canon. He does use martial arts; in The Adventure of the Empty House, the story in which Doyle resurrected Holmes, the detective tells Watson how he defeated Moriarty at the Reichenbach Falls – using the Japanese martial art of ‘baritsu,’ which is probably just a misspelling of the real martial art of bartitsu. It’s fair to say that your familiar deerstalker cap-wearing Holmes (a note: he never wore a deerstalker cap in the books, although in Hound of the Baskervilles he does wear a hat with ear flaps) never busted out any karate.
Of course your familiar deerstalker cap-wearing Holmes fought the fucking Nazis. Basil Rathbone is seen as the quintessential pop culture Holmes, but his Universal Holmes movies eschewed the stories’ Victorian setting and moved the action up to the then-modern day, World War II. In Sherlock Holmes in Washington Holmes is essentially a spy, and he’s all mixed up in a case about missing microfilm. Obviously other Holmes films took plenty of liberties, but most of those are dismissed as parodies; the Rathbone Holmes, though, is widely considered the Holmes, and most images you see of the character are based on this guy. Who fought Nazis in World War II.
Ritchie’s Sherlock Holmes actually does a remarkable job of being in canon. You can pinpoint where in the stories it takes place, as it depicts Watson moving out to be with his eventual wife, Mary Marston, who met during the events of Sign of Four. Holmes’ wardrobe and personal style are also closer to the original stories than any previous version; Holmes lived as a Bohemian given to depressions (and drug use) between cases, and he also dressed in the height of fashion of the day – he was something of a dandy. Rathbone’s Holmes had a stodgy air, while Robert Downey Jr’s has a more faithful anti-authoritarian vibe. Obviously this doesn’t make the film good, but it’s all part of the care that went into making it fit into the canon. 

The place where Sherlock Holmes gets away from the stories is the scope of the adventure, as well as the tone of the tale. While Ritchie et al may have brought the characters back to the original vision, they’ve certainly been set into a story that has more to do with the pulps of the 30s than the penny dreadfuls of the previous century. I think this is a valid sticking point for those who are purists, although I don’t think it’s any worse than transporting Holmes to World War II or ignoring canon altogether to depict his CGI-enhanced adventures as a teen. Whether Sherlock Holmes works for you as a movie on its own is one thing, but the film’s faithfulness to canon can’t be faulted – only your knowledge of Sherlock Holmes can be.