Mixed feelings here. As an OVA goes it is a good ride, interesting and good for extra development that links well to the main story. It covers a good period and gives details that are helpful in the main story.
The art still looks amazing and the sound is great.
The issue with this OVA for me is that it is partly shown in the main show. I think if you watch this first it spoils the main anime, if you watch the main anime and come to this afterwards then it can feel a little bit dull. I did the latter, whilst I enjoyed myself it was just a stretched-out flashback from the main show that I didn't think I needed to have.
For me this is a 5/10, I don't think it was needed in the grand scheme and could have been better for an OVA showing a backstory of the main issue.
