It doesn't really add anything to the main story beside just little things here and there. Mainly adding to their character development and relationship... especially to emphasize why Senjougahara is the best girl for our main boy. Not saying she's best girl overall, that still belongs to our Vampire Queen.
As for this story, it adds a unique element to it. Can't really review this without spoiler because it would make no sense at all how this will continue the series or add anything to the series? This movie basically takes place in a alternative reality or we should say, a mirror world... or at least that's what it wants us to think?
It presents a unique idea of the characters in a different light as if you know, mirror reflect light which is why we can see our reflection, though not 100% accurate. This too is basically portraying that concept and it had a unique idea going for it. For the most part you'll be left wondering what the heck is going on? But it explains itself pretty well as you follow along. I say the first time you watch this, just let it explain itself and don't try to analyze stuff. I would do that on the second time so you can enjoy it better this way.
There's not much to talk about the characters beside a unique twist to their characters. How none of them are as they seem at all. Okay I probably get hated for this but dude I love the adult version of Hachikuji more! I enjoyed seeing her since I saw her in the second season one. So to see her in here again, I was glad to see her. But yeah I can't really say much for characters as you can see how they all have different personality or unique changes to them. And we also see a character who we have not seen before who freaking was awesome! I won't spoil this one...
Overall, I love that this happened though it doesn't add anything to the overall plot. It's kind of like a OVA and movies that's just there to add more enjoyment but not really canon as they kind of poke fun of it in the movie itself. So it was pretty self-aware. Worth a watch for fan of the series.
