Just played through the first campaign, all I can say is awesome. The thrill I got was much more so than what I got with L4D1. Each campaign is different by zombies and objectives, rather than just route and scenery in L4D1. For instance, the first campaigns finale is filling up a car with gas and crashing through zombies and a glass door wtih a race car. IN SLOW MOTION!
All I can say is awesome game. If you liked L4D, you will LOVE L4D2 to pieces. If you hated L4D (like me) you will still love it.