Heart wrenching visual novel. I saw the ONA based on this years ago, but only now got to play this. There's not really much difference, so I think you can take your pick. As for me, despite knowing what was coming, it still hit pretty hard. It's a fairly standard post apocalypse type setting, but the prose is pretty well written, and conveys how deeply fucked humanity is, and how life changing the few days the protagonist had with this robot that should not be working. Just a wonderful little story, fairly short for a VN.