To begin, the story of SaiKano tells the relationship between Shuji and Chise, two high school seniors and their rocky relationship. Shuji is insensitive and tough, but after having to deal with the foil-like Chise who's shy, naive, and traditional, the two form a bond of love. Naturally, uncanny things begin to get in the way of their relationship--Shuji's past love, Fuyumi; Shuji & Chise's mutual friend, Akemi; and of course, a world-wide war in which Chise plays a vital part as a killer superweapon.
Thematically speaking, SaiKano is mature and angsty, but also full of real yet surreal moments and situations that question all sorts of human emotions. Categorically speaking, Saikano is a Sci-Fi love story that paints an unreal world but appeals to real issues. The mixing of pure, unadulterated love with the horrors of war creates an unique and amazing collage of emotions and circumstances.
There's really not much else I can speak about right now, besides the "Go and get started on reading!" :) But for those of you who would rather watch TV, SaiKano is also coming to TV, in a TV animation slated July 2002.