Here's a 20 Minute Interactive XCOM: Enemy Unknown Trailer For You
It's like a demo, but more handsy.
Be sure to enable Annotations when you watch the video above, otherwise it won't make much sense. What we have here is a sort of XCOM Choose Your Own Adventure minimovie. The 20 minute trailer takes you through some example combat with sample squad configurations, giving you a good eyeful of the game's UI and mechanics.
I'd recommend going through the trailer multiple times to see the various selections play out. But don't just watch it straight through, I mean, that's what sociopaths do.
XCOM: Enemy Unknown will release on October 9th for Playstation 3, Xbox 360 and PC across North America, and on October 12th everywhere else. The PC demo, available through the game's official website or through Steam, is already live and waiting for your download. If this trailer doesn't convince you to get on that, boss, I'm not sure what will.