While it may be a GTA clone, it is an extremely good one which will keep you entertained in short sessions.
The story is not very good, but that isn't why you play Crackdown- you play it to have mindless fun, whether it's picking off Gang Members with a Sniper Rifle from the top of a massive skyscraper, throwing cars at them, or doing on foot and in car races.
I have yet to play the online mode but co-op exploring the massive city seems great.
The driving is fun, especially the races and it is good for getting around the massive city. You can also increase you agility skill and jump onto the roofs of massive builldings to get around with nobody in your way.
The enemies aren't very difficult but they are numerous enough to keep you occupied, especially since you can dispatch them in numerous ways.
While it may be a great game Crackdown does have it's downfalls, with the irritating soundtrack and no proper camera movement whilst driving. The great Cel-Shaded graphics and gameplay more than make up for this, though.