It's a good premise for a movie, and an interesting thought.
However I'd have liked it more if Joon was a likeable character. But I find her insufferable. I don't care about her mental problems, because her personality is so awful.
I spend most of the movie hoping Sam would find someone else who I could cheer for.
I wish they'd worked harder on the Joon character so that I actually cared what happened to her. Why put so much effort into Sam, and almost nothing into a titular character? Was Johnny the magic ingredient?