Weak on plot, but fun in pretty much every respect.
The best part about this game is undoubtably your character's running and jumping abilities. Running around the city, climbing skyscrapers, and trying to reach the 500 agility orbs is a blast. As the game progresses and you collect more and more agility orbs (or via several other means of leveling up) your character's ability to run and jump increases allowing you to climb even higher and jump farther.
On top of agility, there's plenty of fun to be had driving, shooting, punching, and blowing things sky high. As your driving skill increases your Agency vehicles obtain new abilities, allowing you to plow through cars, or jump and perform various stunts. Leveling up your strength eventually allows you to pick up huge objects to hurl at your enemies, and leveling up your explosives skill results in progressively more impressive explosions, which eventually will destroy things within a huge radius. Probably least impressive of all the skills in shooting, which is also the slowest to level up, but it is critical for passing the game.
As far as actual tasks go, the game is pretty straightforward and consists of killing 21 well protected gang bosses. Some backstory is given for each with regard to their role in their particular gang (there are 3 gangs), but it's brief, and probably Crackdown's weakest point. A strong plot could have helped make this game even better. Beyond the gang members, there are the optional tasks of collecting agility orbs, and hidden orbs, as well as completing agility races (by running and jumping) or car races. There are some extra downloadable things, and some co-op modes, though I never actually tried them.
There are three levels of difficulty, which is probably about all that can be said for replayability. Once you've beat all 21 bosses, and completed all the races, there isn't a heck of a lot to go back for. Still, what is there is great fun. It's definitely worth checking out.