I can finally say this without spoiling this episode: μ is Hatsune Miku and Aria is Rin Kagamine. We finally get a full explanation of the origin of this fabricated world and its purpose. We know μ doesn't have bad intentions, but we finally learn why she imprisons people against their will (if you like Digital Circus or I Have No Mouth...). This episode has many reflections on loneliness and the ways of experiencing it. It's quite clever that within the debate about Smeagol (yes, from Lord of the Rings), we get a clue as to how both characters cope with their loneliness. I think the only negative point is the resolution of the conflict. It feels a bit ridiculous.