It's fun to be a superhuman. But the rest of the game is pretty shallow.
There are a few other things to do, such as the rooftop running races and the road races. You can also spend an inordinate amount of time trying to collect all the hidden orbs and agility orbs which boost your abilities. Once you slaughter the gangs, there's some stunts and various other little things you can do if you want to collect all the achievements. There is cooperative multiplayer and some other multiplayer modes to tinker with, but with a game this old, don't expect to find much for games online, unless you have a friend with Crackdown.
It's fun being a superhuman who can leap from skyscraper to skyscraper, throw busses, and the like, especially when you skill-up your abilities. But ultimately Crackdown is a little too shallow with it's lack of a story, lack of direction, and repetitive gameplay. If you want an open-world game where you can go around and do pretty much anything you want and have the option of character and story development, GTA is your ticket. If you just want to be a kick-butt dude and blow things up, then Crackdown will fit the bill.