At first, seemingly a slice-of-life about female friendship in a school setting; then it switches to mystery and action with a heavy sci-fi background. And that's what sets it apart from similar manga: the scientific concepts involved are truly intrincate (quantum mechanics, nothing less), and they're thoroughly and extensively discussed by the characters. Unless you have prior knowledge of the topics discussed, you'll probably have to reread the chapters two or three times to get everything. But it's worth it. The plot manages to integrate all the abstract physic concepts wonderfully, and makes them central to the development of the story. In fact, that's the strenght and charm of this manga, the unpredictable, astoundingly solid and fascinating story.
On the other side, the characters, while not bad by any means, are pretty flat and generic, despite the extraordinary circumstances they are forced to go through. Because of them, the manga turns out to be half philosophical physics reflection, half moe loli lightheartedness, but it works, and very well.
Even the weakest thing about this manga is pretty rare itself: the excessively fast pacing. Simply, too much happens in too little time. It's not that it feels rushed, but a lot of juicy events that are going on -some of which could easily have provided material for another manga- are explained away with one or two sentences and a panel. It's a shame, really: if keeping the same quality as the present three, this manga would have benefited greatly from ten or even fifteen more volumes, even mantaining the same story! This is specially bad for the characters: they don´t get enough time to develop properly, and seem to react too weakly to the most traumatizing events, perhaps because these terrible events have only been mentioned in passing.
That last paragraph may seem a turn off, but it's only a complaint about something that could have been excellent, but only turned out great. Do yourself a favour and read this carefully crafted, irresistibly addicting story: three hours or three weeks, when you reach the end you won't think you've wasted your time, I guarantee it.