I find in most shoujo manga the romance part is either too shallow or too childish. Hana to Akuma is an exception.
All I can say is that I thoroughly felt how much hana loves vivi and how much vivi loves her back. That's enough to make this shoujo manga a masterpiece.
The emotions in the manga are expressed just right. For those who have read Mars and felt how heavy the atmosphere is, Hana to Akuma is not that excessive. But it's not like your typical Boys Over Flowers either.

I really like the art and characters too. Vivi is hot and Hana is adorable. There aren't any major character development, but that is reasonable since they both love each other since the beginning. I won't spoil you the ending, but I think you'll find it just right. It's realistic yet heart warming at the same time.