OB is definitely a worthy investment ESPECIALLY if you don't have half life 2 and episode 1. I don't know, but apparently i'm like the only fan of the single player games. Anyways, TF2 is pretty good although i think that critical hit is bullshit. whatever. helps me too sometimes i guess. It sucks when you're stuck with medic 'cause everyone wants to go demo, soldier, or heavy.
Honestly though, i think the BEST multiplayer first person shooter is Day of Defeat. For some reason, i hate games where you gotta shoot a guy multiple times in the face for him to die i.e. CS. Although i will say COD4 is pretty badass