Bachna Ae Haseeno is inspired by a host of Hollywood movies (which the case of most Bollywood films). The film is heavily inspired by "Alfie" (Starring Jude Law) and "The Graduate" (starring Dustin Hoffman). Rumour mills have it that the film also has shades of the old Dev Ananad classic "Teen Deviyan".
Director Siddharth Anand is brain-farting when he says he is personally very proud of the film (Ta Ra Rum Pum). What he actucally means, and smacks of arrogance is the audience be damned! I don't give a monkey's. I loved the film and thats what matters.
A film does well because of good script. You can make a bad film from a good script, but you never ever make a good film out of a bad script.