HL2 / ep1 / ep2 is one of the very few 'single player' games that can keep my interest enough to finish it actually. i thought the game kept me interested the whole way through, and i liked the way the storyline wasnt spoon fed to me all the way through like others.
i'd like to go back and finish bioshock. but honestly, from what i can think of the HL2 series and Zelda games are pretty much the only single player games i finish.
now multiplayer games like TF2 i'll play for years to come.
