I've seen it now!!!
No spoilers version: saw it, loved it, can't wait for season 4!!!!!!!!!!
Spoilers version:
Loved every bit and thought it was brilliant. :)
I figured out something was up when Mycroft was talking about "the virus in the data," that's when I thought, "Oh, this is Sherlock's mind palace for sure!"
Now I really want to know who's behind the "Did you miss me?" thing--is it Janine [Moriarty]? Does Moriarty have a twin? Is it a collective? And clearly Mary is in on something. And Redbeard looks more and more like "the other one." And the future doesn't look bright for Mycroft.
Those are my quick thoughts.
EDIT: MOLLY. FUCKING. HOOPER.