Truth be told: this is only a single episode that cannot keep up with plots that are played out over a whole season - unless you have an overwhelmingly phenomenal main Anime to support it. The OVA "Moments" sketches the everyday life of the young Kousei, Emi and Takeshi, thereby picking up on flashbacks from the original series. Other characters as their piano teachers, Tsubaki or Kaori get featured by short appearances, which makes persons in the know of the series smile several times. And it adds a whole lot of love to this short episode.

In my eyes, you can sum up the episode in one sentence: it is coequal with the main series. And there is no greater compliment I can make an anime than to draw even with Shigatsu wa kimi no uso. The fabulous design together with this awesome music acts out the musician scenes perfectly. With the background knowledge from the main series, the characters come to life without much effort - especially as their thoughts and actions in this episode agree with information we already had. Therefore, I explicitly recommend watching the main series before this prequel!

We have Emi, who cried out loudly when she heard Kosei play the piano for the first time, because she was so moved. Afterwards, she decided to play the piano, since she wants to join this world of music. But when she finally did, Kosei had changed significantly and lost his former easiness. Thus, she was provided a new goal: to get the little boy back who enjoyed playing the piano as much as she does now.
On the other hand, there is Takeshi, who admires Kosei in the same way he does with his Mecha Anime heros. Especially regarding the little fight of his and Emi's teachers, it made me smile when Takeshi "wasted" days just to seek Kosei and get inspired by his way of life more than the teacher could inspire him. It adds on top, that he sees both the passionate pianist Kosei who exercises the whole day, and the juvenile "I'm jumping from this bridge"-Kosei having fun with his friends.
The only negative point I would have to make is that I would have liked to see more of little Kaori, since she is the person driving the main series forwards. However, this may just be the reason: to give the other characters some spotlight and not pick up on the story we already saw.

As a last similarity with its original: how the hell do you direct such a mind-blowing end? It almost hurts to not be able to write down this god-forsaken masterpiece of a last scene as I don't want to spoiler too hard. But if you enjoyed the original, the last 5 seconds alone are worth watching the total episode!

As I still haven't reached the mandatory 2100 characters for this review - a needless trial for such a short episode - let me summarise what I have written before: Moments is most worthy of being the prequel to Shigatsu wa Kimi No Uso! All the character development, the music and the design is on point and adds up to a fantastic overall picture. Having seen the original anime provides you with the knowledge and the closeness to the characters, such that you will enjoy this additional short episode.