Well-told.  The characters are believable, if not particularly complicated (it's a short story, you can only develop them so much!), the plot is painfully realistic, and the art is gorgeous.
A lot of readers are complaining that this manga's a little flat, but I think that it's well-rounded.  It's equal parts character, plot, emotion, and audience interaction.  The pacing is great, because I, at least, am not interested in reading a several-volume epic about the downfall of a relationship.  That's just depressing.  Geeze.  But the mangaka hits the key points - the point of happiness, the start of infidelity, when the partner finds out, when it ends, and finally, a point afterward that carries the manga beyond the relationship, and even leaves the reader with a new complicated thought (which is great storytelling technique).
Easily one of the better manga that I've read.  I give it a nine only because it was gosh-darned depressin', being about relationship failure and all.  It really deserves a ten.