I liked the story. Obviously trying to apply real-world science & logic is not going to lead you anywhere, because of the supernatural & hypothetical elements involved, but the feelings, the relationships, the bondings; oh so beautiful!
Hikari x Manaka works, though Hikari is too brash and Manaka is too dumb at times.
Tsumugu & Chisaki are the most well-rounded characters; the way their dynamic plays out is so sad but sweet. Chisaki desperately trying to cling onto the past, not accepting that she has falken deeply in love with Tsumugu, for the fear of forgetting and forever leaving behind her sleeping loved ones; it's just heartbreaking! Honestly, I'd have loved if they were the main couple throughout the series. They perfectly fit into the real-world love & marriage mould. I'd be happy if there were a spin-off, but I guess it's too late to ask for one at this point.
While Sayu is good, I still feel the most bad for Kaname; having to live as the permanent second, and later, third choice, and KNOWING it, internalising it; oh!
Definitely recommend a watch!