No surprise to anyone that I'm pretty interested in Crackdown... I'm an absolute sucker for open world mayhem games.  We got our copies within MGS a week or so ago and I've been playing it a lot (well, when I get the chance between work, kids, and Warcraft).  It's no GTA but in many ways, like Saints Row, it sets a high bar for GTA4 to meet.  The draw distance is absolutely amazing -- you can stand atop a building and see out across the whole city including pedestrians and far off traffic.  While I was no great fan of the look in screenshots it plays a lot better in motion.

What impresses me most is the co-op though.  You can do pretty much every activity in the game in full co-op and it's fun to just wander the city doing random stuff along with your buddy.  That's not even getting into all the sandboxy fun you can have just inventing your own games and challenges.  Gears of War set a high bar for co-op but I think Crackdown manages to exceed it.  The number of good co-op games has been one of my favorite unexpected surprises of the way the Xbox 360 has played out.