I thought this was so tiresome. Way too 'clever' and contrived for its own good. They can't just let John have an emotional affair, it has to turn out to be with Sherlock's disguised sister...
The serial/cereal killer stuff was boring, went on too long, and was so fucking pleased with itself, and not justifiably so.
Mary-as-John's-conscience is a tired old TV/film trope.
Too long spent on Sherlock mansplaining how he knew about thingy's kitchen, and drawing silly diagrams in the air. Too much silly effects generally, actually, with all the floating texts, the speeding up/slowing down, etc. This show has a huge budget now and the producers just LOVE it – special effects, a helicopter and swanky locations in every episode...
And why so much shooting all of a sudden? Was it Raymond Chandler who said (I paraphrase wildly) 'If you're stuck, have someone run into the room with a gun'? That's all they bloody do these days.
I didn't like Sherlock being explicitly 'off his tits' and didn't really buy that it was to get John to be his friend again. I liked the earlier series when it turned out to be nicotine patches
. Thought the overtness and the track marks etc was all rather sensationalist and grubby.
At some point someone said something like 'Sherlock tweeting? I can't believe it,' as if it was new, but they made a big thing in the last episode about him tweeting, didn't they? 
On the up side, Benedict is HAWT when he hasn't had a shave.