I watched the prologue after the whole series (1st season). And I am glad I did not do it any earlier. The prologue (besides offering a good music video) is not required for understanding the anime. All "mysteries" about the characters will unwind as the series progresses.
The prologue is presented as a brief quarrel between the two female main characters over the person they both like - a main conflict of one of the two stories in the series. However, it is, in my opinion, much more interesting to watch the series without the conflict already revealed. After all, the series is created such that viewers have to resonate and grasp the emotional turmoil of the characters and so gradually appreciate the conflicts.
Conclusion: DO NOT WATCH this prologue before viewing the actual anime!
(The rating scores are intended towards the actual series.)
