if you're looking for replay then i would choose L4D only if you have friends you like to play with
some people like playing games with random people, i dont, i wouldnt play L4D without at least one of my friends playing with me, this game is an online game, you will not enjoy it as much solo
i too have not played gears 2 after finishing the campaign on normal and insane,then beating horde to level 50
now its sitting on the shelf, the campaign was probably the best one i played all year though
it's not a bad game at all, if you never played Gears 1 online then you'l have a high learning curve for online, since it seems that everyone already knows how to play and you'll die a lot, most likely from chainsaws
Fallout over Fable 2, I would say Fallout 3, Fable 2 is a game that is good to beat since its short, but the replay value is extinct IMO, in the time it'll take me to beat Fallout 3, i would've beaten Fable about 3-4 times, after you're all done with Fable 2 that's about it, i didn't see no difference in being evil or good, my choices really didn't have much effect on the world(unless you call people running away the effect) anyways, Fable 2 should be a rental more than anything even with the expansion coming out, i still sold my copy
edit: i was playing 1 hour ago 2am PST, I had no problems finding people to play with
public matches you can just jump in if they lose a player and the cpu is in control of the team mate
i dont see any number telling me how many people are online, but this game looks to have a good following