For me it's the best shoujo-ai/yuri I have ever read. The unique world, where everything took place, was splendidly built by mangaka, the characters were interesting, the stories were touching, the art was superb. It was non-hentaish, non-ecchiish, not depressing yet dramatic, and purely without vague hints yuriish - such a rare case for this genre. I also recommend the anime of the same name (though the anime has only nice hints for shoujo-ai)