what is Bioshock about? *spoilers*

i've been reading some strange stuff about the game, altough i finished the demo and for me, it's just a game like many others
but i wanna understand what this is about

in the end there's a criptic message like someone posted in another forum

Spoiler: Show

"The little sister turned out to be your little sister and you never arrived in rapture because you were always there and it was always an aspect of your mind, but the struggle of arriving was trying to break out of your own insanity caused by atlas, a crazy doctor?"

It was totally like shodan from system shock 2. Crazy man. Unbelieveable. I havent been rocked by an ending this hard since fight club."

obviously i didn't got it

and what about this comment of a member of the team?
http://www.gamespot.com/pages/unions...=13529&print=1

intentions? harvest or rescue? wtf is this about?

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