A fun superhero / GTA mashup of sorts.
I'm assuming most owners of Crackdown achieved that status primarily out of a burning desire to obtain Halo 3 beta codes. Though I can't say for sure, my guess is that those people who took the time to sit and play Crackdown were treated to quite a pleasant surprise.
I hate to compare Crackdown to GTA, but such comparisons are inevitable. You play from a third person perspective; you have access to a city with multiple neighborhoods/islands; you can create ridiculous amounts of destruction in short periods of time. However, the experience is markedly shallower.
First, there's not much of a story to speak of. You're a supercop who needs to take down three gangs in Pacific City. You get little video dossiers on each of the 21 head honchos that tell you whether they handle recruiting or weapons dealing and whatnot, and you're also given a general idea of what kinds of nasty business each gang is known for. This is really a problem inherent with any "GTA clone," it seems. GTA is great because the experience is so organic and story/character driven. Other games do their best to re-create that open world anarchic sandbox feel, but tend to really drop the ball on the rest of the experience. Crackdown is no exception.
But what Crackdown does well, it does very well. You have five core abilities that level up as you use them: Agility, Driving, Explosions, Strength, and Firearms. By the end of the game your supercop can leap about 30 ft, lift objects that weigh about 10 tons, have an explosive kill radius of 50 ft, etc. Leveling up these abilities is a lot of fun, and really makes up the core of Crackdown's value. It really never gets old leaping from building to building, raining fire and death on helpless gangsters below. The only real issue I have in this regard is the Driving. Unlike GTA, driving seems completely superfluous to the gameplay experience here. I have beaten the game with all abilities up to the 4-star max, except driving which rests comfortably and meaninglessly at the lowly 1-star level. I have probably spent a grand total of 3 minutes in a vehicle out of the 12+ hours I've spent playing Crackdown. I'm sure once you get all the vehicle upgrades (which look very cool, by the way), driving is pretty sweet, but that doesn't change the fact that your supercop's super speed and jumping ability is all you'll ever need to get anywhere at any time. And at the beginning, the vehicles just handle sloppily with questionable physics, so I never felt it worth my time to get used to them.
Once you finish the game, you're treated to a narrated summary of the great work you've done, and the requisite plot twist which sets up the sequel... which at this point I don't believe is forthcoming. There isn't much replay value to Crackdown, unless you're an Achievement freak. Co-operative play is a good option as well, except that the host's game progress is used for the co-op world. That means if you've already beaten the game, there's no sense in hosting a co-op match as there will be nothing of significance to do. Conversely, it's relatively fun to join a game where the host is still working to beat it as your character maintains all of his earned abilities, making you an effective aid if your buddy is struggling to take down those pesky guards en route to the target.
Crackdown is definitely worth a rental, and I would recommend it for purchase if you find it on the cheap.