A great story, and the most empowering combat ever
The Force Unleashed is about the apprentice of Darth Vader, the time is somewhere in between the 3rd and 4th episodes of the movies. Throughout the game you go through his struggle between remaing loyal to his master, or listening to the words of the jedi he must destroy.
There has never been a game like this before, and that is due to both the story and the gameplay. The best part of the gameplay is the force powers that you have, as those empower you and just make you feel badass. That alone is an endless source of entertainment, and justifies buying this game.