Why? Well for starters there is the best chase sequence since The French Connection. Then there is Will Smith as an actor - not just a star, though later in the movie he is admittedly overshadowed by veteran Gene Hackman.
There are two layers to this movie: On the surface is a pacy thriller with edge-of-the-seat chases but underneath lies a telling commentary on government surveillance. It is one of those truth-in-fiction stories which makes its point about government intrusion into privacy dramatically and effectively.
There are references to the classic, The Conversation: The surveilled couple talking in the park, and the Hackman character's premises are an obvious recreation of his workshop in the earlier movie. If you haven't yet seen The Conversation - see it before you see this one - you will understand the Hackman character a lot better (besides, it is a superb movie in its own right).
Oh, and Jon Voight is terrific as the bad guy...