Firstly It was indeed extremely steampunky in feel and in plot. There were arcane symbols (although no mention of alchemy that I caught), a filthy and teeming London, with factories and abandoned buildings and new ones being built. People were dirty looking, rather than pristine, And people weren't all beautiful. What a relief.
As to the story, again, not an original plot, but adequate to the vehicle. There were lots of mentions from the book, and I LOVED this Watson. I've hated almost all of them in films to date.
I think Downey went over the top several times, so I'm glad Jude Law kept dragging him back down to Earth.
I'll look forward, though, to watching it on the tube so I can have close captioning on. The interactions between Holmes and Watson were wonderful, but the dialog was fast paced and I know I missed a lot of cool references.
Action packed, violent, dark and interesting. Definitely not the Basil Rathbone sort of Holmes.
Recommend for those who enjoy this sort of thing.
P.S. I can see why Doyle fandom is wildly divided. I think they should embrace it rather than condemn it. But what do I know?