you can play it as an FPS but you wont get very far, in order to actually get very far you really have to use this mode that sorta changes it into a turn-based combat system...and it's totally sick. you can freeze time to target specific parts of your enemies and blast their arms off and shit.
other than that it's pretty much a straight up RPG. it's sick going around to the different ruined parts of DC that i see every weekend in real life.