This has some flaws (is that a surprise?) but it is so much better than its trailers suggested. The pacing is too fast and it feels too modern and Holmes and Watson don't solve a mystery so much as punch and shoot their way through a mystery, but if you accept all that, this is good pulpy fun. Victorian London is full of putrefaction and dark alleyways and snaggleteeth. The art design is excellent, as is the score, maybe my favorite Hans Zimmer music ever. The actions set pieces, and there are several, are very good, especially the one involving an elaborate human trap in a slaughterhouse.
What about its felicity to the canon? It doesn't really matter, of course, because movies don't replace the books they represent. That said, except for the elaborate action scenes and the ridiculous plot of the villain (no idea what was going on there), it's not that far off for Conan Doyle's novels and stories. Holmes prided himself on fisticuffs, was often dirty and drug-dazed, and rarely wore the deerstalker cap. Robert Downey Jr. does a good job, not great. The best performance in the film was Jude Law as Dr. Watson, playing the most-famous sidekick as an almost-equal: intelligent, quiet, and compassionate.