Maybe a little more than halfway through. Minor spoilers ahoy!

So many great characters in Bioshock. The idealistic, power driven Andrew Ryan, the twisted Dr. Steinman, the cold and brilliant Dr. Suchong, Dr. Tenenbaum and her history, the mad artistry of Sander Cohen, the mysterious big daddies and little sisters, minor characters that may become major in time and Frank Fontaine, who is a genius in his own right. The picture is coming together but that can always change.

The audio diaries work great, mainly because the writing and acting is of such a high quality. You gain an understanding for these complex people before ever meeting them, knowing their fate or their true motives. Unlike a game such as Oblivion where I could care less about the supporting cast involved, let alone any of the major players despite interacting repeatedly with them face to face. I know that great characters in gaming isn't anything new but it just adds to the overall experience and makes it that much richer and enjoyable.