Now I wasn't expecting that I would love this equally to the main show's episodes. I watched it after I finished the whole anime and I recommend you to do the same. Watching it like this, it felt like a last "farewell" to all the characters in a very enjoyable kind of way! The OVA had all those elements that the anime holds, even in just this one episode. I found it both entertaining and emotional. I gave a 10 to all aspects as it still has top notch quality in story, art, sound and character's design and interaction. The opening and ending theme are the same as inthe anime and as I wrote on my review about the main anime I love this songs so much, both SID's opening and Kalafina's ending.
Overall a certain 10 from me!