Okay, I'm not going to get a reputation as a controversial journalist by saying things that everyone else has already said, but I played the Xbox Live demo of BioShock at the weekend and was blown away -- and not just in the game.
It's not that easy to pin down what's so good about it, although I have a feeling that it's my love of Art Deco styling and the deeply atmospheric music and sound effects. In a lot of ways, it reminds me of both Myst and Douglas Adams' Starship Titanic, neither of which revolved around shooting deformed underwater-dwelling, biologically altered humans with bolts of lightning. But both had a graphically rich environment, with Starship Titanic being set on an Art Deco spaceship.
I have some reservations about the score BioShock has been given by some review sites. As a reviewer myself, I'd be very reluctant to give anything a 10 out of 10 score -- I've yet to encounter any product that is perfect. Indeed, I've yet to encounter anything in the known universe that I'd consider perfect.
Based solely on the demo, I'd venture this game is at least an 8, very probably more an 8.5 or 9. Until I play the whole thing I'll reserve judgement, but one thing is for sure -- this is the most excited I've been about a game since Half-Life 2.