I played a few recent games of L4D2, and it's a zombie survival masterpiece. The AI director keeps the adrenaline levels high. My hype for B4B is waning. The gameplay footage I've seen isn't very impressive, and the cards seem to add unneeded complexity. Still, I look forward to the reviews at release time.
I went back to watch the intro/preview video for L4D2, and it still impresses.