It is always a problem trying to have a movie with only one or two main characters for its entirety, especially with this one, When Will Smith, many CG mutants and a handful of mannequins combine, while similar to World Trade Centre where two people act in some collapsed structure with lots of flashback, it is hard to sustain any interest without falling into a boring pace.
With the director's decision to make the mutants in CG rather than real humans in costume, it somehow reminds one of the plastic robots in I, Robot starring the same star. Computer generated just doesn't feel as good as a shock as having real characters pushing their physical limitations chasing after him.
The movie loses out to the novel in terms of plot and intensity. You will need to read the novel and watch the movie in order to realise the difference in depth and meaning. And the bitter aftertaste of the movie's ending, with its shallow meaning of the word, seems to distort the word "Legend" that was referred to in the novel. It's sad.