Really fun and addicting. I must confess that at first I found it overwhelming and pretty difficult. Cirno is a tough opponent! I was getting hit all the time, which was a bit frustrating, but after several tries (and switching to Cirno) I started to get the hang of it, and eventually I played through the game several times with all the characters.
Being able to see your hitbox was very helpful in the beginning. I must also say that the AI is programmed very well, and I like how it can make mistakes sometimes, which makes it feel fair. However, I feel it spams danmaku too much -- and in that regard, I think that the shots could have longer cooldowns, but faster speeds, so that they could be used more strategically. But anyway, that's getting into design choices. I'd also suggest making the hit sound effect louder and deeper, so it's clearer when you get hit.
All that said I had a lot of fun with this game; I played yesterday night and I was playing again today throughout the day. The art is great, the writing is very funny, and the music is fantastic. Delicious sax. I'm not the biggest sports fan, but I really enjoy a fast-paced and engaging sports game like this one.
p.s. Unrelated but I want to say that I somehow scored two goals in a row before hitting Eiki and it's a personal record haha