I was definitely expecting something more innocent and fluffy when I went into this, and that was not what I got. There still was a "sincere love story", but there's definitely something...off about this couple. As you continue reading the manga, occasional instances of creepy and disturbing imagery make it clear that there is something terribly wrong with Yuri's reincarnated lover.
Other reviewers, both on this site and others, have given the series bad reviews due to the synopsis being "misleading." Personally, I think it was done very well. It wouldn't have had quite the same impact if I knew what I was going into, just like with Madoka Magica (as another commenter observed).
I do have to agree, though, that the plot was a little ridiculous and unrealistic at times--like when the main character wanted to commit suicide over this guy that she barely knew. And though the romance was there, it wasn't very well developed. Why is Yuri so devoted to Aono, anyway? Hopefully we'll get an actual backstory later.
As for the art style, it's a bit simplistic, but it's easy on the eyes and I think it actually suits this story very well. You'd never expect such a disturbing story from art like this, and that's what makes it better than some of the other hyperrealistic but predictable horror manga. Personally, I also feel the art style made Dark Aono's moments much more chilling.
Still, I wouldn't exactly call this manga a "horror." It's more lighthearted than that; the characters are quirky and don't take their situation that seriously, and the manga is riddled with inappropriate jokes at inappropriate times. Honestly, it's made me laugh more than it has sent chills down my spine. Definitely give this one a shot!