First of all i'd just like to say that if you haven't played through the first Bioshock game to do so as soon as possible before you buy this otherwise you'l be completely unaware of where you are, what you are and pretty much just whats going on around you. The first Bioshock game was fantastic, and Bioshock2 is a very worthy sequal.
Without making any spoilers the game takes place years after the first, and essentially puts you in the shoes of a prototype 'Big Daddy', the hulking sea-diver/cyborg-like abominations from the first game. You are subject 'Delta', a big daddy who has broken free from the control of its master, and woken to find your 'little sister' is missing. The prototype that you are, you have a special lifelong 'bond' to your little sister, and one of the primary goals of the game is to evidently be re-united with yours and help bring down the new tyrant of Rapture, Sophia Lamb, in the process. The story isnt quite as involving and interesting as the first game, but is is nevertheless very well told and keeps you wanting to continue on through the action to reach the end. The combat however is vastly improved and you can now use plasmids (the various genetical enhancements gained throughout the game), along with various weapons both at the same time. There are some fun weapons to be had including a gatling gun, torpedo gun that pins enemies to walls, double barrelled shotgun and more. All of these weapons can be upgraded one upgrade at a time from specific upgrade stations along the way. Upgrades include clip size, damage increase etc, and when an upgrade is applied, the model of the weapon changes accordingly. E.g. increasing the ammo capacity of the shotgun applies 2 extra barrels on the sides of the shotgun which looks neat.
The graphics in Bioshock2 are on the same engine (the Unreal3 engine), but look much more glossy, detailed and refined than in the previous game. I am running this on a fairly average price Geforce GTX 260, a duel core Intel Core2 Duo, and 4GB ram yet im able to play on high settings, max resolution with no slowdown whatsoever.
The sound is also top notch. You can hear splicers (the deformed citizens of rapture) talking amongst themselves before being engaged, the weapons sound cool, the music builds good atmosphere etc etc.
I have yet to check out the multiplayer because im so caught up in the single player still, but have heard it is pretty generic and wont hold people for too long. But i wont acknowledge this as a credible fault since i only really got this for its engrossing singleplayer.
Overall this is an essential purchase for shooter fans who like a bit more to their games than mindless shooting or generic style shooters.