I made the mistake of watching this before I watched gameplay of the game or watched any of the anime, so I was really confsued. I reccomend watching/playing the game or watching the anime before watching this like the other reviews suggest.
Anyway, after re-watching this with my new knowladge, I found that I really enjoyed it. I have to admit, my opinon is a tad biased since my favorite character from this series is Nagito Komaeda, and I'm a huge fan of Danganropna in the first place, but it's still a really intereting story that you should check out.
Nagito is probably one of my favorite characters in the Danganronpa series so him having his own little bit makes me happy but I feel it is really important to mention that you should DEFINITELY listen to summaries of the second game (if you aren't going to play it) and watch Danganronpa 3 before you watch this. Despite this being 2.5, it will make a lot more sense if you do both of those before you watch this. If not you will be lost. The Danganronpa series relies heavily on previous knowledge.