Got it yesterday and finished it in about 15 minutes. It's good and all but....I wouldn't say it was worth the extra 6 months Valve took to get it done.
Not to mention there have been tons of complaints about the DLC for L4D2. I got mine for L4D and I saw no problems other than at one point there was some serious slowdown for some reason. But the L4D2 DLC has had some serious bug reports, like zombies running through walls, players falling through the floor, and things like this. Considering Valve's excuse for taking so long is usually to get everything right, I find this quite funny.
Anyway, time now to wait for L4D3.