I saw I Am Legend last weekend. Due to my less than stellar reception of the book, I was hoping that the movie would be one of the highlights of my film-going season. Multiple reviews I read had compared the vampires in this film to the aggressive zombies of 28 Days Later, giving me hope that I am Legend would be as good or better than that film. Unfortunately, the film was almost as disappointing as the book.
Still, it had its good points. The cinematography was particularly visually stunning, properly conveying the loneliness and noble decay of an Manhattan abandoned by its human inhabitants. While I initially assumed that the film's setting would be relatively boring (your generic Manhattan cityscape digitally decayed by removing all of the people and adding tatters - see any episode of Heroes where they journey to the future), I was surprised to find myself waiting for pauses in the action so I could lose myself in the background.
Just as the Manhattan of the film is more interesting that the drab Midwestern suburbia that is the setting of the book, Will Smith's Robert Neville is substantially more likable than the character found in the book. When we first meet him, Smith's version of Neville is driving around the city with his dog, trying to lead a productive post-apocalyptic life: get gasoline, hunt deer for dinner, wait at the pier for other survivors who are not coming, try to figure out a cure for the disease that has infected most of the world's population and turned them into bloodthirsty vampire-like creatures that ate all of the other human survivors, wash the dog, etc. In the book, Neville is spending a quiet evening at home: drinking whiskey, listening to classical music, and freaking out about the mob of vampire women exposing themselves to him outside the house in a kind of macabre Girls Gone Wild designed to entice him to go outside and be devoured.
While the latter Neville's lack of motivation in face of his more depressing circumstances is understandable, it also prevents the reader from empathizing with him. The film's Neville is a soldier-scientist (important enough to have made the cover of Time) determined to find a cure for the disease afflicting the world, and unwilling to leave Manhattan until he does so. "This is my site," he says repeatedly throughout the film, taking a responsibility for his environment that most of us do not. With his lack of such aspiration, one could claim that the book's Neville represents us as we are, while the film Neville is us as we would wish to be.
This leads to one of the most annoying themes of the book I am Legend, namely that man is a creature ruled by habit. The book's Neville realizes that his lower middle class "habits" of alternatively drinking him to death and wallowing in despair are unproductive, so he procures a microscope and lots of stakes, in order to both better understand and destroy the vampire menace. After discovering that vampirism is spread by bacteria (and is therefore theoretically reversible), Book Neville continues to spend his days killing vampires. After a visit from a vampire who warns him that vampirism is becoming a liveable disease and that his days are numbered, Book Neville still spends his days killing vampires, unable to change his destructive lifestyle.
At the end of the book, Book Neville retains some measure of satisfaction at the fact he was the killer of hundreds, if not thousands, of vampires. Personally, I was a little bit disgusted.
But the ending is also where the film version of I Am Legend ultimately fails. When I think of the film's ending, the phrase deus ex machina comes to mind. From the sudden appearance of other sympathetic human survivors to a cure that appears just as our heroes are about to be overwhelmed, to a voiceover ending that suggests that the cure has ended the disease and Neville is a martyr, having saved all humanity, reeks of that smarmy Hollywood serendipity one expects of sports movies, not horror films.
This film would be worth seeing on DVD, especially if alternative endings (including one more faithful to the book?) are included. Hopefully they will be less fortuitous than the theatrical ending, although I hear that in one of them, the aliens turn out to be vulnerable to water and the protagonist's dead wife's dying words provide inspiration for their defeat.
 In one such scene, a billboard with both Batman and Superman logos appeared, advertising a film that most people assumed would be Batman vs Superman. You could hear excited whispers ripple throughout the audience (who had previously enjoyed a trailer for The Dark Knight).
Wired suspects in its excellent review that it was a guerilla advertisement for a film about the Justice League (supposed to be released in 2010; the year on the billboard). A MTV blog claims that it was just a prank by Akiva Goldsman, one of I Am Legend's writers.
 Like Jenna Wortham, who reviewed the film for Wired (link in the first footnote, above), I assumed that these people would actually turn out to be vampires. The idea that they, like Neville, are members of the 12 million people who were immune to the disease that somehow avoided being eaten by the vampires that outnumber them 20-to-1, is infinitely less interesting.