To make a long story short, Kobukuro are a male and more experienced version of Angela Aki.
This is not to say that Kobukuro aren't fit for ladies: both play the same testosterone-free pop, in which clean guitars, strings, and piano unite to create a sunny, feel-good-movie atmosphere; it's just that Kobukuro is a male duo.
The one thing to differentiate their songs from Angela Aki's chamber vibe is their ability to be over-the-top with the huge arrangements: if her music is made for a romantic trip or a date, theirs is fit for a wedding party (or a movie about a wedding).
The voice of Shunsuke Kuroda is naturally strong, and when the band capitalizes on his skills by employing a big chorus as on " Donna Sora Demo," it's like the "We Are the World" gang singing about love in Japanese, which turns out to be downright impressive.
Besides, Kobukuro employ an array of the best Anglo-American classic pop/rock tricks, such as vintage keys, mouth harp, banjo, and homeopathic doses of hard rock guitar ("Gekkou"), and all this, together with their brimming energy, compensates somewhat for their main flaw, also present in Angela Aki's music: compositional monotony.
Most tracks have no reason to be this long (eight pop songs over six minutes is sheer overkill), especially when each of them sounds largely like the previous one: the band changes arrangements, but never the mood.
Admittedly, though, the positive and romantic mood that they stick to is far from the worst choice, and in the end, even if the songs of 5296 blur into each other, separately they hold the potential to melt the heart under the right circumstances.