So, it's nice to see the backstory out of the way. I must admit, while I do appreciate having both stories there (especially Nana K's, she usually gets a bit shafted, or at least she did in the movie), I found the sudden transition to Nana O's story, mid-episode, to be rather clumsy. I would have prefered them getting both backstories down to two episodes and had a clear transition between them.
Of course, it is interesting now, watching an anime when I know pretty much exactly what is going to happen next. This is the first time I've followed a new release of an anime based off a manga I've read.
As for ruining the ending, I am a bit concerned with that too, since the manga is not finished, but I'm hoping that it's so far ahead of the anime that perhaps it will have been completed by the time the anime catches up to it (I have no idea if there is a planned ending soon though for the manga).