I wasn't convinced of the genius of the original "Crackdown" until I spent 3 or 4 hours playing and replaying the demo that launched a few weeks before the final game. Once I had maxed out my character and began leaping from building to building with ease, I understood the magic of the sandbox shooter. Microsoft must've realize the success of the first demo, as they've gone and released a demo for "Crackdown 2" with extra doodads thrown in for good measure. It's up on Xbox Live right now, available to Gold members.
The "Crackdown 2" demo allows for 4-player co-op, using that fancy drop-in, drop-out magic that was talked about in previews. Even more interesting is that players can actually work to unlock achievements in the demo, and when the game actually launches, those achievements will unlock instantly.
If you're really feeling social networky, you may want to play the "Crackdown 2" Facebook game, "Chucks Ducks 2," which apparently requires Silverlight, so I won't be playing it. But if you do happen to have admin rights on your work computer, give it a shot! Playing the game is supposed to unlock a new weapon in "Crackdown 2" adversarial multiplayer: An exploding rubber duck!
I haven't tried the demo yet, but it's the first thing I'm doing when I head home tonight. Here's hoping all the lag issues have been smoothed out. The prospect of silky 4-player "Crackdown" co-op makes me all giddy.