After coming off a stream of heartbreaking and well-written mangas/books, I felt that Koizora didn't particularly strike any sort of emotional chords in me. I mean, it takes a pretty overused cliche for tragedies AND combines it with ridiculous characters and a storyline that looks as if it's been made to induce tears. With respects to the manga, the art isn't anything great (but it gets the job done), and the pacing is inconsistent.
The characters are barely tolerable, and their actions are unfathomable. Hiro is a major arse who needs some advice on how to treat the one he loves well. I somewhat understand his actions, but what he does to "protect" Mika, is to hurt her (emotionally and physically), treat her like garbage, have relationships with other girls (in front of her) when she's obviously still in love with him, because that's obviously what you do to not hurt loved ones right? How about communicating with her, telling her the truth, figuring out what you guys need to do to get through this time, and having a lovely time before...yea? Mika, on the other hand, needs some major psychological help. With all the crap that goes on with her, sometimes I wonder how she can even get herself up in the morning. I really don't understand how she can still love Hiro after all of this. Comparing her to strong heroines with similar backgrounds like Lisbeth Salander was a HUGE mistake on my part, but really, for a story that so many sing praises about, I expected it and its' characters to be in a league of their own. And they're not.
I think if I had read this a couple years ago when I was in high school the rating would have slightly been different, but I've just seen far too many great tragic mangas/stories/movies/etc... to get emotional about this. I think a lot of appeal of Koizora just depends on where you are in life, so if you're a bit older and want a fantastic romantic tragic manga go read something like Melo-Holic.
*NOTE: I wrote this after I read the novel, watched the drama, and read the manga. *