If you are a Gold Member on Xbox Live (and seriously, if you are planning to play Left 4 Dead 2 as single player or local multi-player only, you are missing out on the best parts of the game), the Left 4 Dead 2 demo is out.
Personally, I’m not going to bother. To me, a demo is something you play if you are not yet decided on purchasing in order to see if you enjoy the game. For example, I played the demo of Mirror’s Edge because I wasn’t sure of the game, and I’m glad I did because it saved me money. But from all accounts, Left 4 Dead 2 is going to be Left 4 Dead only with more awesome. Considering how much I love the original, there isn’t a chance in hell I won’t love the sequel. Valve just doesn’t make crappy sequels.
As a birthday gift this year, I was given a pre-order of Left 4 Dead 2. So I will happily wait for it to show up and not spoil any of the game playing a mere demo. But I won’t judge you if you do play the demo. Have fun!