If we must watch the last man on Earth wandering about aimlessly, it may as well be someone who can hold our attention like the charismatic Will Smith. The actor conjures both pathos and absurd laughs in "I Am Legend," until the film turns from a quiet meditation on the nature of humanity into a B-movie schlockfest. Were it not for the last 15 minutes-it's really appalling, and feels like it's from another movie-"I Am Legend" would have been much more than its pulpy premise hinted at.