I suppose if I had to sum up "The Reichenbach Fall" in five words, encapsulated in one reaction, it would be "I was not expecting that!"
I don't think anybody would be surprised by the revelation that the Reichenbach of the title is Sherlock himself, given the name "The Reichenbach Hero" by the popular press after his recovery of the Turner painting of the iconic falls.
The episode is basically Moriarty following through on his promise at the end of "The Great Game" to "burn the heart out of [Sherlock]" and it is executed marvellously. It was a joy to watch the plan unfold and to see how thoroughly Jim had screwed Sherlock over, enacting a plan that could lead to his own death just to destroy him. Part of the brilliance of Moriarty's scheme is that it goes far enough that even the viewer starts to doubt Sherlock. And it's not unknown for the protagonist of a story to create a nemesis for themselves or turn out to have been masterminding the evil scheme through an alter ego (indeed, in The Last Sherlock Holmes Story and a few others, Holmes turns out to have been Moriarty all along).
If I did have a complaint about the episode, it would be that the rooftop confrontation was a tad disappointing in that it was nowhere near the scale of a visit to Switzerland to fight among the falls. I can see the difficulties in conjuring up a plausible reason for them to go to Switzerland though, so I'll let it pass. Lack of thundering waterfalls aside, the confrontation itself was thrilling stuff and is the standard that every minute of television should aspire to.
This episode has made me completely re-evaluate my opinion of both Steve Thompson, the writer of this particular episode, and Andrew Scott as Moriarty, going from my initial appraisal of him as a hilariously camp man with occasional moments of menace to nothing short of a sinister evil genius. He's not the best Moriarty (that honour still belongs to Jared Harris in the recent Guy Ritchie movie) but he is a very close second.