I AM LEGEND is a great movie for an hour. The last man on earth tries desperately to figure out a cure for the virus that has turned humanity into zombies.
It's amazing how movies with no dialogue are always the most interesting. There is an entire worldess scene played in one long take close-up on Will Smith that should have earned him an Oscar nomination. But then, as soon as another human character enters the film, the bottom falls out completely and the movie degenerates rapidly.
The last act is so weak that I cannot recommend the film. However, the sound design was outstanding. (Again, when there is no dialogue, everyone on the crew gets challenged to do their best work and be their most creative.)