This OVA is a crossover of 3 music bands ("CaS", "d2b" and "DEARDROPS") from different visual novels related to each others, the content of the OVA itself is only a concert (no story) where some of the best songs are performed by each band.
If you know any of these visual novels ("Kira☆Kira", "DEARDROPS" and "d2b vs DEARDROPS") I think you're gonna love it, the feels are all there, for me personally DEARDROPS is something special and just watching/listening my favorite songs performed in an animated concert was fantastic.
All characters are quite unique and lovely. The only flaw of this OVA is maybe the animations, there're some peaks where the difference is quite noticeable, still, there's a very high quantity of details in all close up scenes and with the catchy songs is a perfect combo.
This OVA can be watched before or after the visual novels because there're no noticeable spoilers, but there's a lot of difference in watching it after the visual novels because these songs will bring back the feels, altogether.