At last! I have finally finished the series, with a ten-month delay.
I think the series works only if the viewer would or can tolerance high amounts of melodrama. The plot does explore the rocky-road side of high school life, and I'm pretty sure that not every high school story is pastel colors and smiles all along.
Saya, like all humans, do make mistakes, and her mistake here — manipulating the hearts of innocent girls to do her bidding — ultimately backfires when she brought Yuu even closer to her death when she dragged Mato (and BRS at the same time) into insanity.
Mato, Yomi, Yuu and Kohata also portrays the different "defense mechanisms" that a person turns to when facing painful memories: stubborn passivity for Mato, seclusion for Yomi, hiding in a facade for Kohata and extreme escapism for Yuu. Mato's capability of facing her problems is what made her triumph over Insane BRS in the end, and it was the key in protecting Yuu and the Otherworld from suffering from self-destruction.
However, as life goes, the battle is not yet over for them. No matter which side you look at, there is nonetheless a tough battle — whether in the wastelands of the Otherworld, or in the streets of Real Life.
I rate this show a 9 in a 7-10 scale, for being able to tell a decent story and being straight to the point about it.