Card's story of Mack Street, a young orphan boy growing up the inner city who grows up dreaming other people's dreams, is a real treat of a story. The characters, especially Mack, are well rounded and believable, and the plot is subtle and nuanced. Mack is the consummate "strange" kid, and suffers from waking dreams which he knows arn't his, but are actually the dreams of other people in the neighbourhood. Between these dreams and the fact that he was even alive today was nothing short of miraculous, given that his birth mother had tried to abort him at seven months, Mack has come to believe that his life has a purpose. Most of the rest of the neighbourhood seems to think he's crazy, and suffers from epilepsy, but Mack himself nevers loses faith that there is a reason he's alive today.
What I think makes this story great, however, is that this "purpose" isn't some grand teenage wish-fulfillment fantasy. It's something much simpler than that. It's closer to home, but important nonetheless, and far more beleivable. As to what it is, I'll leave that to you to find out for yourself.