Crackdown 2 really should be renamed to 'Crackdown Revisited' or 'Crackdown 1.5'. The small additions come in the form of vehicles, weapons, enemies and missions. The locations are the same and the gameplay is identical. Adding a lacklustre online mode isn't going to do the developers any favours either.
Having said that, if one were to rate this game from the perspective of someone who has never played the original title, there is no doubt that Crackdown 2 provides plenty of excitement. With approximately 1000 collectables scattered around Pacific City, you will definitely be kept busy for a very long time.