I just completed Bioshock 2 after having it as a dust collector in on the shelf for over a year. I can honestly say that it's one of the best game experiences that I have ever had. The atmosphere in the game is just staggering and there are some cool little gameplay twist at the end that I had no idea about at all. In my opinion I think it's better that the first Bioshock.
So I you haven't played this game yet you should absolutely do that at once!