This is a collection of short stories highlighting events and persons that would not fit in the main story line. There is an extra scene from Amane and Mahiru's visit to his parents, several scenes of Amane and Mahiru's domestic life, and Mahiru's interaction with Miss Koyuki that provides added details to the story arc.
I'm usually not a fan of short story volumes, honestly I skipped volume 5.5 until just recently but this volume was surprisingly interesting for a series of short stories. From what I understand some of the short stories in this volume are referenced in later volumes so it should probably be read for big fans of the series. Overall I enjoyed this volume and I don't think it was lacking in any area.