The beginning had me immediately drawn in! The diversity and the chemistry between the two Nana's and their developing character/relationships were really great!
But as the series continued, it just got worse and worse...
The main characters became less and less likeable, especially Nana O., character development was rather forced or unexplained, and the motives of the character's actions were never reasonable or explained unlike in the beginning of the series. And to top it all off, the writing got more sloppy and unfocused as the series continued; sometimes even getting the chronology of the story wrong!
I haven't read the manga, yet I'm very certain that it is better than this anime. So in case you haven't seen the anime yet, I HIGHLY recommend that you read the manga instead!
