This 4 episode arc is brilliantly plotted and contains some of the most clever dialogue writing I've seen in anime. The deliberate pacing of each new reveal was perfect for me, and the fact that the story still felt cohesive despite many of the main characters being in separate locations for the majority of the arc made it all the more impressive. Mustang was the star here, and got developed a great deal beyond his simple ambitious personality. The depiction of his masterful plan was flawless, and all the twists were truly effective due to the great writing and direction.
In this episode we get our first glimpse of the profound depth of his relationship with Riza Hawkeye, which obviously extends far beyond merely subordinate/commander. Al also gets some time in the limelight. The stability of his soul bonding being called into question gives some additional urgency to the brothers' quest outside of the homunculus/military coverup. The climax of the arc in the Third Laboratory with Lust was emotional and thoroughly satisfying. Voice acting from everyone was exceptional! The animation was vivid and visceral in a way we really hadn't seen in the show thus far, and frankly it yielded some of the most iconic shots in anime. Despite this I did feel the pacing of the episode was a bit rushed. Also the rather juvenile "Lust has big boobs" humor unfortunately diffused the rapidly building tension in one particular scene. I would much rather have seen her embody her titular trait through subtle innuendos and snarky dialogue like she did later on in the episode. Just another instance of poorly timed/executed gags breaking immersion for me.
So while this specific episode wasn't perfect for me, the arc as a whole was fantastic and shows how great FMAB can be. It displayed smart writing, authentic world-building, great character development, and continued to augment the underlying mystery and tension of the plot. 9/10 episode, 10/10 arc.