Absolutely masterful.
In my opinion, SnK/AoT's major strength lies in its plot. The plot is what attracted readers and the plot is clearly what the author focuses on (and, in my opinion, sadly at the expense of characters who remain rather flat and who I have little attachment to).
This arc brings an already original plot to new heights. The mystery that has been fostered for hundreds of pages is finally resolved and the answers bring even more depths and layers to the story.
The plot twists are mindblowing, nothing short of incredible and kept coming, one after the other. The payoff is that much stronger thanks to the way the author has planted the seeds of his revelations since early in his story. It makes you want to go back and reread everything from the beginning; actually, I think that it is obligatory to go back and reread everything so as to see all the details and work that the author pulled off.
Plus aside from the plot twists, the story in itself was thrillingly good this arc; from the last gallop to Armin vs Berthelot and Levi's choice... Excellent stuff all around!
If there was ever a counterpoint to the go-as-you write Stephen King style, I would say it's this tightly plotted marvel that was delivered to us by Isayama!