I've read the novel, and I loved what they were doing with it right up until the third act, when the lady and the kid showed up and the movie turned into 28 Days Later. The movie largely went down in flames for me at that point and started making zero sense.
And I hated hated hated the CGI "zombies." They really didn't do anything that screams "We need Gollum in this shot," so I don't see why they couldn't have gone for a practical effect with actors in makeup. It pulled me out of the story every time I saw one of those pixelated predators pop up on the screen.
Will Smith is awesome in it, though.
This is going to be one of those movies I'll appreciate more if I don't watch the ending on future viewings... sorta like A.I., where I stop the DVD when the screen fades to black as he's staring at the Blue Fairy so I don't have to put up with the tacked-on alien garbage at the end.