This is definitely the series strongest point. Although it starts of slower then most animes but picks up quickly.The story is interesting and is very "deep", tackling many subjects such as existence, isolation and other. The story also makes effective use of symbolism and foreshadow, such as the sky, the night which the audience may not recognize at first but will realize along the series. Its these attention to details and little things that make the story exceptional.
It is also worth noting that this series has one of the best endings ever, it is not a generic "happily ever after ending" which has been done to death, not much more can be said without spoiling the story, however the ending does not leave the audience feeling sad or "empty" as it wraps up all its loose ends and reveals everything to the audience.
The art is great, the action scenes are beautifully choreographed. However, my only gripe is that all the characters had a very triangular chin, which i can't stand.
Voice acting is wonderful, especially the voices of matsuri and Aono. The theme music employs a light piano melody, which isn't spectacular by itself, but when combined with the animation onscreen, it really comes alive.
Characters in Sola may seem empty and lifeless at first, however once the audience watches the entire series, this will be explained and the audience will realize what a fantasic job the directors have done. Character design is also exceptional, especially Aono and Matsuri
Overall, Sola is something truly beautiful. It is hard to tell you how so without revealing the plot but I am 100% sure anybody who watches this series will be deeply touched by it's wonder story.
