I was irritated about the marginalizing of Crackdown as a "Halo 3 beta pack-in", but I'm sure the cross-promotion hugely helps them both, so I'm fine with it now. Not that anyone asked.
For those who don't know the genre, an open-world or sandbox game lets you wander around the game world, doing what you want, how you want, when you want.
In this case, you can take out the first- or second-level baddies in any of the 3 gangs in any order you want to weaken the kingpins, or you can skip levels or even go right to the respective kingpins.
Or, you can spend all of your time exploring, leveling up your character (almost RPG-like), getting more powerful on the agility (fun faster jump higher), firearms (quicker and more accurate), explosives (big ol' damage fest), strengths (pick up a car with someone inside and throw it), or driving (faster and better handling).
Or, you can run around trying to collect all of the "Agility Orbs" (there are 500 of them, and I'm at ~370) or Mystery Orbs (there are 300 of them, and I'm at, uh, ~40)
You can also create all sorts of your own mini games within the larger game. Like "Volkball" (get a giant sculpture globe through a goal). Or Lightball (basketball), Rocket Tag, Urban Surfing, King of the Hill, Kick the Car, Car Golf, Huge Air, The Runs, and Jerk Sim 1.0. Or, you can just pile as many cars up as you can, and see how many blocks wide the explosion will be. Or you can pile up pedestrian bodies and see if you can walk up them to that second story (or, um, so I've heard).
Achievements are a big part of Xbox Live, and Crackdown has some of the more creative. I like to play through a game without knowing what the Achievements are (to avoid spoilers), and it's fun to get surprised by things like "Shot-putter" (10 points for "Throw any object (other than a grenade) 205 feet or more") and "Mad Bomber" (15 points for killing 500 gang members using explosives).
Online co-op play is a lot of fun in this game (and needed for a lot of the minigames above). It's fun to work through the mission mode with a buddy. And I'm bummed co-op isn't available offline -- a serious shortcoming that would give the game more legs for me (I'm a social gamer, and online doesn't count for me). I also wish there was more multiplayer, and though there's a version of it enabled by the game keeping score of player versus player kills, I'm hoping this speaks to something more formal down the road. And having 4-way co-op with fully leveled agents would be amazing.
Sandbox games often struggle with a cohesive story, and this is one area in particular where Crackdown struggles more than others. There's no story per se, just "kill all of the kingpins". This is unfortunate, because there's actually a decent story hook that gets laid on you as you finish the game -- which I'm hoping speaks to a sequel or downloadable content.
One of my pet peeves with RPGs, on the other hand, is when you can't replay the game as a leveled up character. Crackdown suffers from the same thing, and I'm not sure why Realtime Worlds made that design decision.
Something that hampers the fun of the game (and a lot of the mini games) is inconsistent in-game persistence. I can be stacking explosive barrels and cars for a mega explosion, but if I get too far away from the pile, things start disappearing. To be fair, this may be a consequence of Microsoft mandating to developers that they don't rely on the Xbox 360 hard drive being present. On the flip side, Oblivion seemed to get this right, and that's an older game.
The one thing that I don't enjoy about the game is it's nigh impossible to play the game without hurting innocents. If I want to play through the game as a "hero", it's wicked tough to do it without collateral damage. Lots of collateral damage.
Overall, a fantastic game, with a lot of longevity if you're creative and have an online friend, and hopefully there's incoming downloadable content and a sequel in the works.
Would I rent? Yes.
Would I buy? Oh, yeah!