This review is for those who have read the main story and are wondering whether this continuation is worth their time, probably in part because it is rated as being considerably worse. For me at least, it was very much worth the read for various reasons, some of which I will try to explain briefly.
First let me say what these stories are not: they do not give any considerable progress to the story, hence probably the low score (at the time of writing this, anyway). Many stories have a good ending, especially in anime, where we are left with a complete conclusion to almost every
event and character development in exactly the best way imaginable. That certainly leaves a nice feeling, however if one enjoys the story enough to imagine the characters as actual people, he is inevitably left with the question: and then what? And, if everything is tied together perfectly in the ending, the characters cannot continue living in a hypothetical continuation of the story since their worth ended with the story.
This story, however, as these short stories prove, can live on forever. Sure, probably neither the novel did nor these extra chapters will give you exactly what you want as an ending (since we all know what that is), but more of the dynamic of the relationship between the two main characters is what they will give you, and in my humble opinion that is the best part of the novel as well. The contrast between the coldness of the main character and the strong emotions you feel burning underneath the ice, combined with great writing, is what makes this dynamic interesting to follow. For this reason the story deserves never to have an ending, but to live of as an eternal build up. Regrettably these stories are indeed too short for what I described, but they are long enough to immerse you in them and give you an hour or two of enjoyment, which nowadays is worth a lot, so don't miss out.
The stories are too short for it to be sensible to give a summary. Sufficed to say: if you enjoyed the part of the interactions between the two main characters which highlighted the more intimate part of their strange and reserved relationship, you should give this a chance. However, if you are expecting any significant development, you might be left slightly disappointed. But it's only two short stories anyway, so you won't be giving away too much of your precious time.