Can't stop playing it, Crackdown is a very very fun game.
You start as a regular cop in a city overwhelmed with crime. Quickly, you start to find new weaponry and secure “supply points” for the Agency (Police).
Gradually, the skills you use improve – driving, agility, shooting, explosives and more. But when they improve – wow, it's like super powers and they only get better. Before you know it you're leaping through an awesomely designed 3D world. No restrictions, you can go anywhere, do anything.
Whether you want to find agility orbs to max out your skill, or hunt for secret entrances to the crime lords' lairs, this game is a blast. I'd played the demo, which I'd give about 3.5 stars and was expecting about 4 stars for the game – but the continuity, the slower pace of advancement, and larger game world that the full version offers makes this 5 stars for me. And I haven't even tried co-op yet …
How does it compare to GTA? Less storyline or character development, but way faster pace, more exhilirating game world. And you get to be the good guy, with extreme prejudice. When falling off those tall buildings … the only game I've played that actually makes me feel like I'm falling!
By far the most fun I've had on my 360 so far. (I've enjoyed Dead Rising and Gears of War too.)