I liked this! There were a couple of little things that kept me from giving it 5 stars: The Sherlock/Mycroft relationship, and Sherlock in general, is a bit too friendly :P.
As other reviewers mentioned, I think that Watson was a bit too slow at the end.
It had a bit of an action movie feel, and a large body count for a Sherlock story, but Watson's narration made it clear that this was both unusual and distasteful.
I didn't feel like the steam-punk aspects contributed to the story in any meaningful way, but neither did they detract from it, and they were a fun bit of set dressing.
I'm interested to see where the rest of the series goes :).