What drew me to the world of two girls named Nana? Well, it helped that the series was translated in Finnish and available in libraries, so I could pick a copy, and it was mentioned to be rather down-to-earth for those of us who are getting a tad tired on mechas, magic girls and martial arts type of manga.
Nana is another of those odd-couple stories, two strongly different characters who by chance are thrown together and who get along despite their differences or possibly because of them. Blond Nana, usually called Hachi in the series, is a cheerful airhead ruled by impulses, gets to be the main point of view for large part of the series, while dark Nana, surly and ambitious rocker, opens her point of view much more slowly.
And to be honest, if the series had just stuck to the characters as they were in the first book and we had just wacky adventures of Nana and Hachi, I wouldn't have bothered to continue reading it for both the hyperactivity of Hachi and surliness of Nana rubbed me the wrong way. Luckily, this series does much better than that.
These characters actually grow during the series, and their relationship as well as the relationships of the supporting cast do evolve. Some become more mature, others start to fall apart...and there's a refreshing lack of easy answers, less-than-ideal situations can turn into blessings while dream fulfillment might end up in misery, and it's quite hard to decide who actually are on the side of angels.
These characters ring true, in slightly poisonous soap opera way, but still.
There's lots of good discussion about the first twelve books (which, incidentally, is as far as I have read the series so far) on Comics Should Be Good website.