These are the sorts of fan projects that leave a big positive mark on the community. Tian Nya is at the helm here, but a lot of others in the community have gotten involved in helping to put this together. Though I'd even recommend this game to others not necessarily into Holo because of the gameplay.

On that note, the gameplay is a mix of shoot 'em up and bullet hell, with lots of projectiles flying across the screen; constant mobility around the entire screen is encouraged if not necessary.
Unlike some bullet hells, it's not so reliable to stick to near the bottom of the screen the majority of the time and learn movements that weave through large elaborate patterns. Enemies usually spawn in at random locations and will move everywhere, some will charge you, some will explode, some tend to kite you with their attacks among other things. Attack patterns do of course exist, but they'll mix and clash with others to create unique threats. So while you won't run into situations where you'll see more raw projectile than the stage's background art, the unpredictability of your enemies is not to be underestimated.
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Managing your energy for your ability to dash and use your ranged attack matter a huge deal. Three attack styles (High damage-cap melee, Ranged & mobile, and Defensive) exist and cater towards different styles of play that can be swapped mid-battle to account for different situations. Parrying is a thing; I'm terrible at it.
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There are a good selection of relics and abilities that can be synergized to great effect, and they allow for different playstyles to shine through. You have a select few that you can start a run with, but you can pick more up along the way as you complete stages. Unlocking these things are based on farming resources from the bosses you fight and the zones you find yourself in, but the grind is not too great.
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If you're looking for a casual time, the game has assist options for auto casting, target lock, etc.
If you're looking to no-hit clear bosses and get sweaty palms, the game rewards you with resources, unlockable outfits, and probably enough bragging rights to help you sleep at night.

As far as fan service is concerned, the pixel art is nice, the character profiles especially. Their weapons and abilities make sense and refer to a lot the of lore and jokes built across years. The dialogue, which can make or break immersion, is fairly well done and not too bombastic while still being amusing.
Then major props to Tian Nya for tackling the task of adapting a fair selection of Holo music in a cross midi x retro style which fits how the game looks.

There's more to get into, but overall the game feels pretty polished despite it still being in early access. It runs well, and hasn't crashed on me yet. The game doesn't try to get too crazy with mechanics while still having the necessary package for a game of its type plus a bit more.
This is my first time playing any kind of 'bullet-hellish' game, to test myself beyond just watching people online. Am I doing as well as I thought? Well, maybe, actually. -but the game is still a good challenge for me without breaking my soul. Even if you're a shoot-em-up and bullet hell giga-pro, this game will likely still be fun to go through and it may even hit you a couple of times. My only real nitpick is that a few of the many pretty forms of projectiles can at times be a bit hard to see among the flurry of others and some backgrounds. My own skill issue? Probably.

Is it worth the price? Depends. As a bullet-beginner, this game is indeed fun and it will last me a while. I believe the end product post full release will be worth it. If you're bullet-pro 5000, it may not last you terribly long in its current state so you may want to wait for a sale or when more content is completed.
If you're a Hololive fan, well. What's another few bucks?

TLDR: Read a shorter review, go on.