I urged my amigos to download and install the game. Just to be sure not to miss out on this great deal.
Btw. wanted to post this thread myself yesterday but GOG just didn't let me (keep throwing 404 at me).

Is L4D2 online only...or is there a Single Player campaign? I might not get it installed by 10am PST, and if it's online only I won't care as much.

I was only getting 56k download this morning, before having to shut it down @ 12% completed.

There is a single player campaign but it's like practice mode for co-op really.And I think just starting the instalation is enough to get it added to your account.

Single Player and Multiplayer use the same campaigns. You can go on the game, go on single player and play as any of the characters available and the rest are bots/AI companions. You can also play the same campaign online. Including L4D1 campaigns, there are in total 12 official campaigns (each have a certain number of maps, like some have 4, some have 5 etc). Theres also another campaign called Cold Stream, not sure if that actually counts or not though.
There are also lots of different user created campaigns so if you do enjoy the game, theres plenty of more to play through.

In my personal opinion, I find playing L4D2 alone boring but if you do have friends that play it, it certainly is a great game with lots of content.

It's still free at the moment.

Yeah, apparently, I wonder if they extended it a bit due to demand. I tried for a bit yesterday, but the page was continuously crashed.

I have no idea if I'll like playing it, but it is free.

"*Offer ends December 26th at 10AM Pacific Time. "

So there's still 1 hour and a half to grab it if you didn't already.

I've been trying it out in single player and my first impression is that I can't imagine actually playing it online with people I don't know, they would hate me, I keep accidentally shooting my teammates.

If you want fun coop with people you don't know, play Killing Floor.

Fun game well worth getting for free.

9gig and stop at 2.3 and never return, i will leave it maybe it will start again in some moment

Really? I didn't see that. From what I've seen on r/games and r/gaming, it was all melancholy regarding the server problems in both cases.

Has been downloading for 3-4 hrs still at 10%. Hope this works.

seems the free time is over. But got this: http://i.imgbox.com/adc9TJxy.jpg

All I did was click "install" in the store to add it to the library. The freebie is over, but the game is still there.
Now to never play it.