There are so many well thought out, valid and educated commentaries already about this stunning masterpiece. A few great ones to check out: Monkeyvoodoo, Littleyellowdress and Geese1's.
This quiet, poignant tale about the human condition touched me deeply as the narration flashed back and forth from significant memories to future choices. The tension was almost a living thing as the reader anticipates the two main characters' moves. The secondary characters' lives all enhance and compliment to explain the motivations of the primary.
If ever there was a manga that opens up education and university-level debate, critical thinking or discussion, this is it. The mangaka has a dramatic way of creating a thousand words with one picture. Intense. For example, when Nanoka says, 'why can't we love?' other than being moved to tears, I was also moved to think. Using genetic problems as the reason for banning laws regarding such intimate matters is lame; we don't ban women over 40 from having kids, and the risk is just as high for genetic problems...
This manga also is brilliant at educating readers regarding incest: It's called Genetic Sexual Attraction in scientific circles. Freud said that we are attracted to those we recognize as similar to ourselves --more trust worthy, prettier, get along better with because of personality traits, etc. He also said that society's taboos evolve to prevent incest. Westermarck said the opposite: we biologically evolved to not be attracted to our siblings, but only those whom we 'imprint' with from early life. He said we have no need consensual bedroom laws; social laws are mere morality exercises in control and domination. This explains incest between siblings who meet later in life if you combine the two theories. Westermarck's theory "out-Freuded Freud" because Freud may have been erotically charged toward his mother, but he had a wet-nurse, and did not imprint with his mother.
Who says manga is not informative?! North American culture downplays the significance of 'comics' as being valid and viable commentaries, equivalent to serious literature.