I just finished it after reading nonstop for about 3 and a half hours. I couldn't put it down. :komue:
I would start off by saying that it's a very "Key" VN, which is probably why I liked it so much. Each character gets their own little arc as the story builds towards the finale, some slice of life happens in the beginning-middle, tears are shed by everyone at the end. (Or, if you're Tipi, pretty much the entire time.) There were scenes and characters that felt like specific callbacks/copy-pastes from older Key works, too. I can't be the only one who saw shades of CLANNAD in the film Madd's son made, and Madd himself is, like some have already stated, pretty much Esaka Lite. Also, this may just be my bias talking, but I saw shades of Komari in Shiona, especially near the beginning. Always smiling, inexplicable dialogue quirks, blonde hair, a goal to make everyone happy... let's face it, Shiona would be an avid supporter of the happiness spiral theory.
But that said, I don't know how fair it is to compare it directly to Planetarian. Sure, they have similar settings and share the Kinetic Novel format, but at their cores they are very different stories with very different atmospheres. I feel like Planetarian put its emphasis on atmosphere and worldbuilding, while Harmonia spent more time on its characters and plot. That's not to say that either is completely lacking in the opposite, but they do feel like two very distinct experiences. And I don't think that's a bad thing! I would rather experience a brand new story than watch Key try and recreate a game they made over 10 years ago.
As for the characters, while I do wish that we could have seen a bit more backstory and development for some of them, particularly Shiona, I think for a short kinetic novel we got an okay amount. This isn't, say, Clannad or Little Busters, where they had over 50 hours to examine every intricate detail of a character's life and personality. They had a story to tell, they gave us the necessary information, and left the rest to our imagination. I would love if we got some side materials in the future of some sort that expands on things a bit, whether it's character-specific or expansion on the world as a whole, but for now I'm satisfied enough.
So overall, there were some aspects that felt rushed, a few cliches I'm sure we all saw coming, and other nitpicks, but overall I'm pretty pleased with Harmonia, and mostly think it was worth the long wait. (For the first time I found myself paying close attention to sound effects, just to hear the effort that was apparently put in.:kurumu:)