http://www.mangahere.com/manga/i_s/
From the man who brought you Video Girl AI, DNA², and Zetman (not to mention he's the character designer for Tiger & Bunny). Katsura Masakazu Sensei!
Great read. Although this manga is recommended for those who have strong resistance to their emotions (or those weak to their emotions as well) because, really, Katsura-Sensei has honed his ability so that his characters have life, making them so that you can feel what his characters are feeling.
In short, this manga will pull at your emotions so much, making you Sad, Happy, Aroused, Annoyed, Angry, etc. at their rightfully appropriate scenes