First FPS I've played in a long time that I've really enjoyed...
The gameplay of the game is solid, but that's not what really impressed me. The story is simply amazing. I would argue that it is probably one of the best developed stories (and "mythologies") I've ever seen in a game. The fact that it is packaged into a FPS makes it even more impressive. The makers of the game created a whole world in Rapture that is sheer creative genius. The characters and creatures are unlike anything I've every really seen, and for something that's so incredibly far-fetched, they made it almost believable.
The gameplay, while not what impressed me most, is also worth talking about. Unlike your typical FPS, you aren't just able to shoot guns. In Bioshock, you can also get involved with gene splicing, which will allow you to harness certain other powers (electricity, fire, freezing... swarms of bees). This is what takes its gameplay to the whole next level. It allows for a lot of character customization, and also will affect what you're allowed to do in the game. It also really adds into the replayability.
The other part of the game that I thought was really cool was the way that you were given a lot more options. It's not as linear a game as most shooters are. Yes, there are objectives that you need to fulfill, but it gives you a certain amount of leeway in how you will complete some of them. It also has the choice of harvesting or saving the Little Sisters... both of which have their advantages, and eventually will affect the way the game ends...
All in all, an amazing game, one that has definitely earned the buzz it created and will be played again (numerous times) by me in the future.