I'll just start off by saying that I wasn't a fan of the first Left 4 Dead. It was a solid game, but it just didn't seem as fun as other shooters at the time (particularly resistance 2). For a game built on co-op, it didn't seem as fun as Resistance 2 or Gears 2's co-op modes. It was still a fun game and had its moments. I just felt that it is overrated by fans of Valve.
Now, going in to L4D2, I had low expectations, and they were met. The game play exactly the same as L4D, with a few add-ons here and there. I felt somewhat disappointed that they're throwing this game out in a year with minimal improvements. It honestly seems like it can all be DLC.
The guns are slightly different, but the core gameplay is exactly the same. Everything feels the same, so if you were a fan of L4D, you will feel at home here. Character movements feel a little bit slower in this game, but other than that, it's all the same.
It's pretty fun when you're playing with friends, but is it really worth the money? Short answer? No. Long answer? Because this is a very small upgrade from L4D and it doesn't warrant a $60 purchase. If you absolutely loved the first one and all your friends are ditching it to buy this one, then I guess it's okay for you to buy it too.
I just feel that the game doesn't offer much more than the first one. However, smacking infected with a guitar is pretty damn fun :D
Overall, I would suggest this only to hardcore-est of the hardcore L4D fans out there. The PC version is definitely the way to go here.
I hope Valve doesn't pop out new L4D's every year like COD.
Anyway, this is just my opinion. Feel free to disagree/flame me in the comments.
Overall, I think the game is a nice solid FPS and a ton of fun with friends, but IMO it doesn't really deserve your hard earned $60 ($50 on PC).