As the title reminds you of the male lead of the romantic saga Heer-Raanjha, we are invested (only) in the male lead of the film too. It works only for Dhanush to launch himself in the romance-obsessed film industry, but fails for other reasons.
Raanjhanaa will be remembered for its flawed characters; but will be easily forgotten for its flawed writing (barring the outstanding dialogues; and beautiful use of songs as narrative device, Aise Na Dekho in particular). Though the director's ambition is uncompromised in this love story, it lacks clarity of thought in its storytelling.
It falls in the could-have-been-better zone, for me.