This game is extremely frustrating for the following reason: There's just not enough to do. And there COULD have been.
I love the graphics/art design. I somewhat like blowing up stuff and killing gang members. And I LOVE to collect me some agility orbs (462 out of 500 collected...something like that). But the game can be beaten in 8-10 hours and after that there's not much left.
I've hopped on top of trucks that have been speeding down the freeway and just shot out car tires, and I've thought to myself: There could've been an awesome car chase scene in this game. There could have been interesting boss battles that required ingenuity and different tactics (see: God of War, although that's a COMPLETELY different type of game...however, since Crackdown is weighted so heavily on boss battles, it deserves mentioning because Crackdown's boss battles suck). There could have been a better, more integrated story to help you care about the characters. (As it is, characters like your agent are literally interchangeable).
Crackdown gives you a great city, and a lot of "could have beens" but not much else. It's still fun, but you shouldn't spend $60 on this game. Rent it for a weekend, beat it, and move on with your life.
Oh yeah, and it comes with Halo 3 beta...that might be worth it, depending on how bad you want it.