Story: Very solid structure and plot. The story can be broken down into a very simple paragraph, but the execution and writing is way more than enough to make up for it. The dialogues are crafted in a way that doesn't feel manufactured, but perfectly expresses the characters.
Art: Beautiful art. Consistently good and almost nothing to pick on. The character's expressions and body language are handled in such a delicate manner that you can tell their exact thoughts without words: something I have never seen any other series do quite as well. It also has very interesting paneling and perspective shots that fit the situations perfectly. There are subtle details rich of meaning that can easily be missed if someone were to simply skim through the pages.
Character: Lovable characters through and through. I'm thoroughly invested in the main characters, and all the side characters are likable in their own rights. These characters feel like real people reacting to real situations. There is no one in the cast that I dislike at all.
Enjoyment: I swear every time I re-read the series I fall in love with it more. Never has a manga really evoke this much emotion from me, let alone a romance. I have no idea how this did not strike an impression on me on my first read, because now I'm surprised there isn't a cult around this.
Overall: I wouldn't hesitate on calling this a masterpiece, please go read it, then re-read it two hundred times afterwards. Oh yeah and be careful of translation errors if you read it online.
