So far I've played through Dead Center and Dark Carnival and I'm impressed. The experience is incredibly well tailored and it is considerably more challenging that Left 4 Dead 1. My personal favourite bit would probably still be the very first part in Dead Center when you are escaping a burning Hotel; it's very cool. Coming across the Gun store later on was a pretty neat buzz, especially since the game limits your weapons in the early stages. The new creschendo bits work well and can really cause some chaos.
Dark Carnival is great too, no real standout bits aside from maybe the Roller Coaster which on the first runthrough got everyone killed. The finale is quite cool too. There's a number of carnival games that are still operational, my group stopped for a while to use the strength test game and play some whack-a-mole; just pointless but really neat touches.
It does feel like a new experience and I was doubtful. I mean the shooting zombies part is the same but the challenges and the environment just mix things up enough that it doesn't just feel like an add-on to Left 4 Dead 1. The new infected seem to work really well together and especially on some of those new creshendos they can really fuck you around.
I guess the real test is whether this game stands the test of time and obviously I can't theorise as to that. I know it's a better game than Left 4 Dead1, (and not just superficially either.) I'm pretty sure it will take over from Left 4 Dead 1 to a large degree which is a shame. Still, Left 4 Dead 2 is a pretty fun title. IF it was the first Left 4 Dead gae to come out it would be worthy of some considerable excitement, as we've kind of seen a lot of it before in Left 4 Dead 1 and that's toned the hype down a bit. Still, I think this game is very good. I'm not going to say that it's a must play because it depends if you're into the kind of gameplay that Left 4 Dead 1 offered - and it is the same stuff, it's just considerably better. If you're skeptical or think that there's no point after Left 4 Dead 1 but are still kind of interested I'd say it's definitely worth picking up.