At first I thought the art was a bit lazy since I'm so used to reading mangas of great detail. But that doesn't matter because the content of this stuff is immensely touching.
It's amazing how it can be so touching without being cheesy, corny or tragic (and tragedies, while interesting, are super depressing).
For a Shounen Ai, both protagonists are masculine and are attractive - basically boyish boys.These guys are equals yet they have completely different personas. It's both an opposites-attractive and alikes-attract.
Zhan Zhengxi's reactions to Jian Yi are perfect. It not only brings comedic relief from what is on the verge of becoming sappy but it says a lot about their relationship. Zhan Zhengxi's feelings to be completely unknown is what keeps this manga going. It's a mystery, it gives the reader something to hope for and gets us constantly looking out for any hints.
From a first read of the early chapters, there is no dialogue whatsoever that explains where this story begins. Because it isn't necessary. The author throws the reader into these collections of short, mundane situations and instantly we know what the story is. And then the past slowly unravels to provide more to discover.
What's so great about this is that there is not going to be a possibility of a disappointing end. Whether or not they end up together is not crucial. And also, it's strongly hinted that the events of 19 Days is a prequel to their adult lives.
It's a unique art style, a unique relationship and a unique story. It's brilliant.