Disclaimer #1: I never saw the first movie.
Disclaimer #2: I'm biased by means of being a huge fan of the Grenada TV Holmes adaptations (the ones with Jeremy Brett as Holmes that still play on PBS sometimes).
Disclaimers aside, I was somewhat less than impressed.
It's an okay movie, but it was repeatedly failing to live up to my expectations. Robert Downey Jr. plays Holmes as brilliant but comically batshit, with the script and cinematography emphasizing his photographic memory and ability to MacGyver together insane plans on the fly. Jude Law's Watson is the grounded one who tags along despite an ever-present sense of being Fed Up With This Loony. I prefer an eccentric but dignified Holmes, which this movie certainly doesn't give us. I do credit the script with not falling into the common trap of making Watson an idiot. Instead, he's intelligent and competent, a fairly good investigator on his own.
The big problem is, the best Holmes stories are intellectual puzzles with only a smattering of action. But Guy Ritchie, true to his roots, throws in one setpiece after another. Of them, only the Cossack fight and the train sequence are truly memorable, and both are fairly early in the film. Meanwhile, the actual case to be solved fades into the background. We know Moriarity's planning something, and we eventually find out what, but it's more a matter of following the trail until his hideout is found then any test of deductive reasoning.
Also, do not make your entire mystery revolve around a MacGuffin which isn't even mentioned before the climactic revelation. That's just cheating, and it's especially annoying because one or two shots of the book earlier would have been all we needed.
Also also, Stephen Fry's fat ass in a birthday suit is not funny. It's traumatizing.