Spoilers ahead for anyone that hasn't played the game yet (you should, by the way).
I wasn't really sure what I was expecting when I started the game, but I ended up enjoying the experience thoroughly. Finished it in around ~45 minutes, but I didn't feel like I'd rushed it; I am a quick reader.
I was pleasantly surprised by the depth of the characters; considering the relatively short length of the game, and the lack of backstory provided regarding them, I felt I had a reasonable understanding of who they were and what they wanted; minus Haze, that wasn't clear to me until near the end of the game.
This might sound silly (I don't play Visual Novels very often at all), but I didn't know the backgrounds/sprites moved between "levels"/"scenes". So when I was reading along and the eyes of the characters were moving, I kept looking up going "Did those move? I think they moved... Nah they didn't move", and then felt very silly when I realized they were in fact moving. Anyway, I don't know if minimal movement is the norm or not, but I liked that the movements (such as eyes, eyebrows, smiles, etc.) when talking with Haze were very subtle, vs. the other character whose name I've forgotten because she's not important! Ending 3, if you couldn't tell. The subtle movements fitted the sad/melancholy theme perfectly, and made the big movement reveal at the end all the more impressive and unexpected. The art itself was great, very impressive; I wish I could draw/design like that, well done.
When the choices came up, I found myself sitting back and pondering for a few minutes, weighing up the pros and cons of the decisions; all except the last decision, that Demon is staying here, no pondering time required. It's not often a game gets me to stop and think like that; I won't lie, I was torn choosing between Ella and Haze. One might have offered a loving relationship at best, and a dysfunctional friend-zoned one at worst, while the other would have offered a proper friendship and their wisdom at best, and possible death/torture/other bad 'Demon stuff' at worst; it was a tough pick, but I was glad I picked Haze in the end; I think he needed Willow just about as much as Willow needed Haze.
I thought the mental health aspect was very tastefully applied; some games take it a bit too far, but you hit the nail on the head I reckon. I was very much expecting Ella to walk up to the lake and go "Ok why are we here?", which could have sent Willow into some deep spiral as she questioned her sanity, what's real and what's not, etc., but no, Haze is real, and in that moment it solidified to me that Haze was acting with Willow's best interests at heart, and it wasn't just Willow's own mind toying with her. I might be reading into it a little too much, but I wonder if Willow was so accepting of Haze as a direct result of her mental struggles? I was worried that Willow was a little too trusting of Haze, in that she didn't seem to question if Haze was real or not, which in itself spoke volumes regarding her mental health and added another layer of complexity to the character, but, alas, Haze was real.
I started writing this with the intention of leaving a review, and it seems to have just declined in me writing my rambles on the page, so, hopefully you find something in here useful.
TL;DR: Good story, good characters, themes well applied; definitely worth a playthrough.