Even though there are RPG and adventure elements, BioShock is at its heart a first person shooter so action fans will not be disappointed at all. Your character fights his way through the many different parts of Rapture and deals with a motley crew of once human creatures that have now been mutated by Rapture's own genetic coctail called Adam. There are a number of different enemies in the game. Most are variations of Splicers, ranging from simple zombies and armed crazies, to ones with special abilities like the Houdini Splicers that can teleport. The main creatures you deal with are the Little Sisters and Big Daddies. The Little Sisters are pretty crucial to your survival. Depending on your moral choice you can either kill them and take all their Adam or save them and take only a portion of Adam. However the Big Daddies protect the Little Sisters at all costs and it takes a lot to bring them down. We found the game's AI to be pretty solid when fighting enemies; sometimes when they were down in health they would try to get to a health station to get recharged. There are also mechanical enemies like stationary and helicopter turrets.
Fortunately, BioShock gives the player a ton of options to not just arm your character with weapons and powered but with lots of other abilities as well. You don't get a ton of weapons in the game (pistol, shotgun, machine gun, grenade launcher, crossbow, napalm launcher, etc), but the weapons have different ammunition types and can be upgraded as you progress. The result is a wide variety of different ways to bring Splicers down to size. You can pick up different weapons and ammo via dead enemies, through searching inside various cabinets in the game or buy them through the game's vending machines (and yes you find money around the game as well. The game also has what are called U-Invent machines that allow you to take what look like useless things from corpses (screws, rubber hoses, etc) and make new ammo for weapons including some that can't be made anywhere else.
The plasmids are the other way that BioShock gives the player a number of different options. These special abilities are unlocked via collecting Adam from Little Sisters and then opening up slots to use them at various genetic vending machines in the game (and yes, we know that sounds funny; BioShock has a ton of humor in it that is understated but still there). The plasmids themselves are found all over Rapture and are powered by injections of Eve, a chemical that you also find all over the city. Offensive plasmids range from typical electric and fire attacks to firing a swarm of insects or telekinesis. However there are four other plasmid tracks that offer special abilities to improve your health, aid in hacking attacks (more on that later), improve melee power and more. While there is a lot to keep track of, BioShock does it with a minimum of fuss. The game's user interface (handled in the same 1940's style of illustration as the game's setting) makes keeping track of all of your various plasmids a snap.