Enjoyable execution of a great concept.
I beat Normal to Lunatic without much trouble, but Nightmare seems very difficult. Most of the patterns become almost impossible to dodge for me, although some can be can be cheesed by staying at the bottom of the screen (the bullets despawn before they reach you), and some are fairly easy to dodge legit (the green "Oo." lines of bullets that bounce once, and the slow lines of diagonal rice).
Disregarding the hard to dodge danmaku, Nightmare difficulty is great in that it forces you to enter the nightmares, while lower difficulties can be beat by mostly just playing the sim aspect well. In that sense, Nightmare is the most fully realized marriage of the two different parts of the gameplay.
Also, some patterns have bullet scaled to a small size that seem to randomly kill? And the small bullets being rendered behind the large bullets isn't great. These are minor problems, and most patterns (on lower difficulties) feel fair and well-designed. I like that the game natural makes you repeat patterns, so you can feel your improvements as you play.
Also, it would be nice to have a more clear cue for when the dream-icons switch, as this can be a matter of life and death on Nightmare. Like a sound effect and clear visual queues towards the characters that suddenly have the wrong books on them.
Also, I think running should be the default outside of the dreams too, rather than requiring shift to be held.
So I listed a bunch of stuff I didn't like, but I that list is exhaustive. Everything else is great, and all-in-all it's a fantastic game.