The vibes are immaculate, the artwork is gorgeous and evocative, the music is lovely and melancholy. The fireflies are a super clever way to help show where you haven't been yet (although I do think you shouldn't be able to collide with them, they start to get in the way after awhile, but obviously you want to grow your adorable little train).
I managed everything but the 4x4 puzzle and couldn't solve it after at least 15-20 mins, which is a bit frustrating because I wanted to see the end! I'm not a big puzzle game person though, so maybe it would be intuitive to someone else.