Well, having got a few golds now i think i can do a deeper analysis on the survivor mode.
Now before i get onto survivor id like to say they actually changed alot, and frankly, i love it. Worth the wait? maybe. Either way, everything is good, and havent found a problem yet (apart from i havent played versus yet...)
Anyway, survivor mode, it's like a panic event. Only that it lasts indefinately, and lots of infected come whether you like it or not. And I guess the reason why I like it is, oh yes, 5 smokers? 4 hunters? 3 tanks? at the same time? YES! You basically get a full set of arsenal, sumthin like 10-20 molotovs and pipe bombs, 8 pills etc... They do not respawn, what you see is what you get. (also some explosive barrels maybe...) So basically, yes there are 12 maps, but they are all areas which have panic events in them, for example the power generator on NM2. Now the only problem i see with that is on NM5 we had to press thebutton twice (as you do in campaign) which is a pain.
I should also mention that I said ive got gold, which basically means I wont be playing that stage ever again. :/
Id also mention that ive done it with 2 friends and a bot, whether the bot gives me the advantage i dont actually know, either way ive proved it's possible.
Anyway back to game -- if you havent got left 4 dead, now is the time you get it!
sidenote: survival mode *is* difficult regardless of whether I have gold on the first day. I should say this because my friend and I have always been pro and killing infected, and its these skills you need to survive. None of that closet camping rush shit, you're in a square and zombies are comming, good luck.