Originally planned to make a review once I 1CC'd the game but I didn't manage to do that and got distracted by too many things and games later on so I'll just make it now.

This game is a ♥♥♥♥♥♥♥ masterpiece. Not just as a shmup, but as a game in general.
It evokes a constant sense of exploration and discovery through the complex stages, enemies, mechanics, and most importantly, its playable characters, which total 7, which is a ridiculous number when you consider all of them are pretty much completely unique, unlike most games of this genre where different characters tend to share more or less the same range of abilities and controls. Not only do you have to take some time to learn how a character controls and what its abilities are, but you keep discovering uses to their abilities as you keep playing with them.
The catch is that, for this reason, it is quite hard to get into, and is going to put you off unless you actually take your time to explore the game (and if necessary, read up on it, particularly the manual). Most games of this genre tend to be "move with arrow keys, shoot with Z, bomb with X" so of course getting confronted with this complexity can be scary at first, but in truth, it is no different than games with complex mechanics in other genres, it's just that shmups tend to have relatively simple mechanics.

If there's one aspect I could mention that would make it move inviting, it's the difficulty. Though learning how the game and the characters work is hard, the game itself is not as terribly hard as other games of the genre can be. Depending on your character, there's a lot of ways to slow down and delete bullets, and to make yourself invincible. Bombs work with a meter that is constantly charging itself automatically (and you can have 3 uses per level of autobomb), and you can get lives relatively often, giving you plenty of room for error.
Additionally, there's a mode which lets you play less stages than the full game, but still beat the True Final Boss, and therefore beat the game, so you don't need to do a real 1CC in order to see all that the game has to offer.
Of course, all in all, it's still quite difficult and you will need to play lots of times in order to get things right if you're not that good at this kind of thing (like me).

Now going back to actually reviewing the game itself, let me talk about the aesthetic.
You may have guessed from the screenshots what the game sort of looks like. It has this strange techno look to it with this cryptic and esoteric, psychedelic, sometimes even religious feel that pervades its entirety, with a weird techno soundtrack to match each stage and unify the game into a cohesive whole. Some things like the character, bullet, or item sprites might look kind of wonky and too game-like and break that cohesiveness, but overall it has a very unique aesthetic that you probably have never seen before, and likely never will again.

On the topic of music, that is one of the main aspects that really elevates the game for me. It's not just that the music itself is good, but how the music is utilized in the game. By this I refer to how the flow of stages, or the timing of the entrance of enemies/bosses, or even the attacks of bosses themselves are often arranged to match the music, practically making the game into a musical with beautiful choreography, and creating more of that cohesiveness. Showing examples of this would probably ruin the fun, but I'll just say that Stage 7 is a prime example of this, and the entire final boss sequence is simply a work of art.

I must mention the playable characters again to leave it clear just how incredible they are.
In this game there's barely any straightforward character that you can just shoot with, they all have different combinations of key presses and key holds you can do (using the same amount of keys you use in a normal shmup) to use different special moves and skills, so you can have things like a character with around 8 different moves using just 3 keys.
These moves relate to the gauges in the left side of the screen, with the behavior of these moves and gauges changing completely between each character.
The game is all about learning how the characters work (though generally you'd have your own character of choice, like a fighting game) and exploiting their abilities to your maximum advantage, finding in which situation each move would be appropriate. This is mainly what gives it an incredible depth and keeps you constantly discovering things, as you keep getting eureka moments for the different ways you can use each move.

This game also features some quite complex story and lore, of which I honestly don't know much about, but it's the kind of stuff where you have to spend some time looking into all the pieces of dialogue spread around everywhere and then come up with your own theories as to what the hell it all means. It certainly has an impressive amount of text for a shmup, I'll tell you that much.

I would love to write a wall of text about more details of the game, especially the actual mechanics, of which I haven't talked that much about, but not only would that be a huge amount of information, but I also believe it's best to discover as much as you can about this game yourself, since there's many, many moments of genuine surprise to be had if you don't spoil yourself.

Conclusion, if you like mechanical exploration and discovery in games, and are willing to put some hours into learning how the game and its characters work (be it via in-game tutorial and trial-and-error, or by the manual and online guides), I highly recommend this game, you'll find out why it is regarded as it is, and will be rewarded with an amazing experience, which is hard to replicate.
Did I mention this game was made by a single guy? Yeah.

PS: I made the only guide on Steam for this game also meant for beginners check it out wink wink