Six Half takes the old 'amnesia' formula to new places because the protagonist, Shiori, is one of the strongest shoujo heroines I've ever seen. She's also very conscious of her own feelings, and since we're privy to her thoughts, even though she may act in callous, childish ways (such as to her supportive brother), I sympathize. Shiori is rare in that she's a dislikable heroine whose amnesia has made it plain to her how dislikable she was-- and this self-awareness, in turn, makes her into a LIKABLE protagonist. She's very human, too, and multidimensional. Yes, she's strong, but the overwhelming circumstances she finds herself in would bring down most people -- she has to deal with her growing self-hatred, her friends have deserted her, her sister hates her... even she would feel alone. Her resolve to change herself into a better person is admirable; this kind of self-fashioning goal isn't a common theme in shoujo, but it perfectly illustrates, to me, why she's a strong character.
I'm only on chapter 5, but so far I'm giving this manga a 9 for its strong beginning; I imagine that it'll continue to get better.
And am I the only one who would be totally fine if A-chan turned out to NOT be her blood-related brother? Because Shiori's lost her memory which means her personality's different...plus I'm wondering why she's a blonde (looks that way) while her brother and sister have dark hair. Ah well~ Kai seems like a nice guy.
Ah, as a final note -- I like how (as of where I am now) there doesn't seem to be a huge emphasis on romance. Right now, it concentrates on the heroine's struggles.
EDIT: Caught up to chapter 11, and OH MY, I cannot wait for the next update. I can't remember the last time I was eager for more chapters...