If ever there was an adaptation of a book I would consider the best, it would be this. You can keep your Harry Potter, I would go for Sherlock Holmes any day...everything to do with Holmes. The books, the films and the TV series. This film is no different. It is one of the best and most accurate adaptations of the franchise yet (tied with BBC's Sherlock in terms of accuracy) and it really shows that they did their research of the books.
The narrative for this adaptation is that Lord Blackwood (Mark Strong) is arrested by Sherlock Holmes (Robert Downey Jr.) and his sidekick Dr. John Watson (Jude Law). Blackwood is sentenced to death for messing with the dark arts and hanged after a threat to Holmes. Days later, Holmes and Watson are informed that Blackwood is back from the dead and continuing with his dark arts and more. While on the case, Holmes almost meets his match against Irene Adler (Rachel McAdams). Adler is trying to seduce Holmes in order to get information about him for her master.
Like I said before, accuracy is the key to this film. In early television adaptations, Holmes is always portrayed as a gentleman, smoking on his pipe while wearing his deerstalker hat. If one were familiar with the books you would know that Holmes is actually a drug addicted, brawling sociopath (I believe Sherlock put it best: "I'm not a psychopath, I'm a high-functioning sociopath. Do your research"). Robert Downey Jr's Holmes perfectly portrays this idea and even goes above and beyond to bring out an accurate interpretation. Same goes for Jude Law's Watson which is also accurate.
Mark Strong is excellent as Lord Blackwood (who is made up by the way. Blackwood wasn't in any of the stories). He creates a really dark atmosphere wherever he is and the fact his emotion rarely changes really makes for a creepy villain. It's just a shame he was pushed aside by the villain in the sequel, but his replacement was still great (notice I didn't say his name? HA! Suspense). One could argue that he's too fictional to be a real person, but I say that it makes him an even better villain. I love villains that use brain over brawn. They are more interesting and three dimensional.
Sherlock Holmes is one of the most accurate adaptations I have even seen in a film. While the plot is made up and not derived from any stories (except a bit from Scandal in Bohemia), the characters used and how they are portrayed complete the idea of what a true Holmes adaptation should be. While people may be turned off by the dark atmosphere, I would recommend the watch the series Sherlock first as it can ease the audience into the accurate portrayal by Benedict Cumberbatch and then Robert Downey Jr.
An excellent adaptation that while it may not be for everyone, is a must see for Sherlock Holmes fans.