The movie acts as parable for American culture and is an almost cautionary tale of our obsession with the self-made man. While we are shown an intimate portrait of the theatre, the theatre itself is a microcosm where the drive to succeed is viewed under a microscope by those who have felt its sting. For Eve her success as an actress is her only measure of worth and she finds no value in others; they're merely tools to an end or obstacles in her way. Of course, someone as manipulative as that is bound to find their equal--in Eve's case a Machiavellian theatre critic, Addison De Witt. The sympathetic characters of the film have achieved success, but without the neurotic focus and drive of Eve; while Margot slips from star into growing obscurity during the film, she still has the relationships of those she loves around her. In sharp contrast, Eve reaches the pinnacle of her career only to appear exhausted with her life (Eve is left intentionally opaque; mostly viewed through the eyes of other characters the viewer often must choose to interpret her actions and guess at her reasoning). The emotional cost of reaching seems a heavy price, but once you've sold your soul to the devil there is no going back and the modern Mephistopheles that is Addison De Witt has every intention on collecting on his star, Eve.
Critical viewers will also note the clever staging and repeated visual themes within the movie. For example, the presence of mirrors in key scenes (from Eve's first introduction to Margo and her friends backstage to the first cracks in Eve's character as she poses in front of one with an appropriated gown from Margo's closet and even the final scene where Phoebe bows to an imagined audience while holding Eve's trophy) is more than casual chance. The mirrors reflect the external focus of those main characters often caught in the reflection; Eve is obsessed with her image, not her physical beauty (it would be pithy to write off the characters as so shallow and basic as that), but acclaim and recognition.
There is a good deal more that could be said of the film, but like most good movies it would be better to just watch it than to talk about it...
All About Eve, film stills by me
All About Eve, film stills by me