Versus shmup that feels the closest to a traditional arcade shmup out of this type of game I've played (compared to say Twinkle Star Sprites which is more puzzle-like and has a lot of unique quirks + neogeo slowdown). Single player is pretty solid too and there are difficulty modes to match any level of player.
Multiplayer games especially on the highest difficulty can get really hectic with the bullet patterns resulting in fun improvised dodging. Highly recommended to fans of the genre if just for this.
Netcode works nicely with input delay not being introduced unless you're controlling a boss form. You can easily play across continents with little issue, and bosses having their control nerfed is honestly justified for most of them.

There is an equipment system which sounds bizarre but in my experience overall makes the game better. The baseline ships feel a bit underpowered to clearing out enemies. While if you are playing with powered up shot + return bullets (these cause enemy kill chains like TSS, or deal tactical damage to bosses!) you can get up to crazy enemy patterns as the game keeps getting harder the longer you survive. Super fun in PVP matches.

For the past few years there has been a japanese & international group playing online on PC every Saturday @ 9pm JST for a few hours. The longevity of that group should be some proof to this being a hidden gem of a game.