I am by no means a Holmes scholar, but I did read a collection (I think the complete tales, Volume 1) some years ago. By my faulty memory, here are some things they got right:
Holmes is an athlete--many film versions show Holmes as the brain and Watson as the brawn, but in Sir Arthur Conan Doyle's stories, Holmes outclassed nearly everyone in intelligence and strength.
Irene Adler is a real character, and did outfox Holmes (I believe twice)
Things they got wrong:
Holmes and Moriarty (minor, unimportant spoiler) do tangle several times before Watson's engagement.
Although Holmes did use coccaine, and Watson did express concern, Holmes was far from an inveterate drunk during his down time, and his drug use never became a problem.
Things they got...different:
This is an action movie, while the books are far more cerebral.
I don't recall the books ever getting into the supernatural as much as this movie did.
Anyway, as for the quality of the movie itself--I liked it. Robert Downey Jr. is perfect as Holmes, Jude Law plays off him very well as Watson, and the action is engaging and fun. The caper is about Lord Blackwood, a very evil man who is put to death at the conclusion of Holmes and Watson's final case together
If you think about Guy Ritchie's ouvre (minus SWEPT AWAY), you'll see that with LOCK, STOCK, AND THREE SMOKING BARRELS, SNATCH, and ROCK 'N ROLLA he's really been making Sherlock Holmes action flicks for a long time, just with different names and more criminals. (And if you think about SWEPT AWAY, you should punch youself in the brain until you stop*) So he's a perfect fit here, and I'm looking forward to the inevitable sequel (latest rumors--Brad Pitt as Prof. Moriarty)
Oh yeah, and while I was there I also re-watched WHERE THE WILD THINGS ARE at the Bear Tooth Theatre Pub. I have nothing to add to my previous review other than my adorable five year old neice liked it, too, and didn't find it scary. Oh, and I liked watching it while drinking beer and eating nachos.
*I've never actually seen SWEPT AWAY, I just heard it sucked. But I couldn't resist a snarky dig.