A great shooter different from your everyday run of the mill shooter!
The other part that makes it feel like a multiplayer is that the game is split up into "maps" kind of like playing a multi-player. Rather than having your standard, clear a room, move onto the next room, clear that room, move to the next room, it is more like you are in a big section with boundaries and you don't move onto the next big section until you have accomplished all of your goals for that section. So you are kind of running back and forth and all around in the section.
I like the idea of collecting money and using the money to purchase your ammo, health, etc. That can make it very challenging.
I think Bioshock is a great game and I am definitely looking forward to the sequel due out later this year.