If you are looking to see this movie because you enjoyed the novel written in the 50's (also titled "I Am Legend"), stop right there. This is not the novel. This is nothing close to the novel. Once you've established that, this movie can be rather enjoyable.
The movie follows Robert Neville (Will Smith), a doctor, who happens to be the last man on Earth. Everyone else is gone, including his family and friends, having been wiped out by a terrible virus. Are they dead? You only wish they were dead. People affected by the virus turn into violent zombie-like creatures who starve for blood and burn in the sunlight. By day, Neville searches for food and plays with his german shepard. By night, Neville hides away in his house while the hundreds of virus affected humans bang at his walls and attemped to work there way inside.
The film is unsually sad, and if you are easily depressed, I suggest you do not see this movie. There are a few cliche 'SURPRISE!' moments, but they are mixed up a little bit and definitley manage to get you jumping. Horror is absent in 'I Am Legend', and the zombie-like creatures hardly manage to put any feeling of dread in you. The visual effects are fairly plain, however the sets are absolutley amazing.
See 'I Am Legend' while you can, and if you miss it be sure to check it out on DVD.