I don't know. I actually went to see it without ever having read any Sherlock Holmes stories (except for The Hound of the Baskervilles in high school, and I don't really remember it). I enjoyed it, although Holmes and Watson seemed quite a bit more like Batman and Robin or a pair of ninjas than anything else. It prompted me to check out the entire collection of canonical short stories from the library, which I have been enjoying for the past month, but I can hardly say from reading them that they movie was anything like it. Honestly, Watson is much MORE competent in the film than he ever seemed to be in the stories. Holmes's cocaine addiction and general reclusiveness is played up an extraordinary amount. And Irene Adler, like most Holmes fanfiction, is given a much larger role (and a role much more romantically linked to Holmes) than she ever had in the canon.

So my verdict: definitely worth seeing, and I thought it was a good movie, but it's very much the Star Trek 2009 version of Sherlock Holmes.

And just so you know: I'm Gaffer Tape! I do my research! You all suck! Mwahahahaha!