Robert Downey Jr. embodies a dusty, decadent, devil-may-care Sherlock Holmes, which is perfect for this action-packed movie. Jude Law makes a very modern Watson, with nothing of the lackey-ish attitude usually attributed to the character. He's a fine and even counterpart for Downey's Holmes, often exchanging funny repartees with him, and a fierce fighter.
Besides the main pair, there's the seductress Irene Adler played by Rachel MacAdams, a clever resourceful thief, who outsmarted even Sherlock himself several times. And there's the villain, the ambitious Lord Blackwood, who wants to rule the world through the Free Masons.
To me, this is the first decent Guy Ritchie movie (Ritchie fans don't get offended!), and it has some smart fight sequences, special effects and a beautiful and properly dark photography. Late 19th century London is shown as decadent, dirty, messy and dangerous, which fits the plot very well. I bet this movie will have a sequence, which is already hinted at in it, when they mention the archrival Moriarty.
If you haven't already seen it, do it!
PS: Sherlock Holmes was the creation of Scottish writer and physician, Arthur Conan Doyle. Doyle wrote many books, not only novels, and was curiously eccentric. He was a Spiritualist, member of several paranormal research societies, and was involved in the ugly scam of the Cottingley "fairy pictures" of 1917, which doesn't make his beliefs less intriguing.
Just as it happened with several famous characters of the Victorian period literature, such as Alice and Peter Pan, Sherlock Holmes started out as a dark character, who was a drug addict, at a time when the use of drugs was almost as common as that of smoking. (Check the interesting article by Mike Jay about it in the Darklore magazine.) Below a selection of sites where you can find more information about Doyle, Spiritualism and Sherlock Holmes.
Official site: sherlock-holmes-movie.warnerbros.com/