Simply put 'Nana' is an anime about love and relationships, not just between a man and a woman, but love and relationships in general.
People are complex, and there are few animations out there that capture that complexity so well.
This anime has been around for a while and should definitely get a cult status.
On the down side 'Nana' tends to get overly motional at times, which might make the watching heart-wrenching; it does get unnecessarily tragic, but hey, our lives are quite often unnecessarily tragic.
The art work could have been better as the characters are unproportionally tall sometims (compared to the furniture), however, that might have been the drawing style at the time.