What makes this so great?

[Andrew R]

And is it a fairly lengthy game? :)

[Unabsolutely]

There's... a few points. The game blends roguelike genre and shoot them up well. Adjustable difficulty provides players not experienced in shmup genre with a great entryway into the whole concept. The game is filled with unlocks and stuff so you always have goals to work for, usual 100% completions being hundred+ hours. Many players resonate with the world, characters and the overall (unfinished, early access) storyline. People enjoy the art and the soundtrack. For more hardcore players there is challenging stuff to be done.

It's hard to say that anything in specific makes this game great, but of many people i've seen each player resonates with their specific part of the game and enjoys that. So, I guess, there's something for everyone in there.

[Mew seeker]

What, my Steam review wasn't enough?
I suppose if you want to the artwork and screenshots pages, you could get a good feel on what makes this game great but they are so full of spoilers. XD

This game is greater than the sum of it's part.
If you look at each part individually like the gameplay, the artwork, the plot, the lore, the music and so on,
you can find others games that does those better, if only by virtue of there being only one person working on this game for several years now.
Even if you consider that this is a boss rush shmup roguelike, a rarity as far as I know, is this really enough to push this all above everything else?
However, when you put all those element together you get quite the package deal.
It all fit together so well creating something so special.

So anyway, in this game you bring love to monster because they leave in fear
When you help them, they give you gifts so that you can help the others as well.
However, they forget after each day so it has to be repeated, hence the roguelike structure.
Similarly to the Binding of Isaac, the game expand as you keep playing and you'll get new item, new monsters, new characters, new game modes, new final bosses and so on.
However, the game just keep expanding in unexpected ways, it's crazy. XD
Stuff that you'll unlock in one mode will start appearing in another and so on.
At first, when you meet a monster, you get a few antagonistic lines before the fight and after they are nicer
but eventually you learn more about them and they're not just things that you come across, they are actual people.
It's super wholesome.

What we got so far implemented for the story and lore is partial but good.
On a surface level it might initially seem cutesy and perhaps even childish
but as you play you eventually realise that the characters have lot of baggage.
This isn't an uuwuu tale about a naive little girl magically solving everything with the power of love and friendship,
this is the tale of people who, in spite of the injustice, pain, suffering and prejudice they received made the choice to love anyway
and I expect the final version of the story to punch the players in the gut a few times.

As for if the game is lengthy, well, if you just want to beat it...
I'll take a "The Binding of Isaac" lens and say that if you just want to beat it, you just clear all the ending.
In that sense, I presume there'll be at least 20-25 ending spread through 2 modes to clear if not more.
To get all the characters and modes needed, the main choke points so far is unlocking enough monsters and getting enough Tetrids (the game currency) to access everything needed.
I'll throw some numbers and guess 20-30 hours for the final versions but who know at this current point, this is still early access.
But if you want to unlock everything...
I had 100% achievement after over 400 hours of gameplay IIRC
but not anymore because Veyeral added an event recently. XD
It's not as bad as The Binding of Isaac Repentance but there is a ton of content and lore to unlock.

Also, you won't have a good grasp on what the game have to offer with only 1 or 2 hours of gameplay. Sorry.
The first run have an initial lengthy tutorial battle to teach the players the main basic gameplay mechanics.
Then, there are various smaller tutorials added to introduce the next mechanics and various items types as you play.
And you can't just go on auto-pilot through them because Veyeral is sneaky enough to put characterisation and story in those. XD
Also, there are a bunch of cutscenes to view for the first few runs.
If you can't skip it, it's not new even if it might look the same at first.
And then, you just keep unlocking more and more stuff. XD

Ah, I also suggest having a look at the option menu somewhere in there.
You can turn on a bunch of useful meters and change the control type for your charged shot if needed.

Hopefully, this answer your questions.