The short: Apparently, it's impossible for Hollywood to do justice to Richard Matheson's wonderful book by the same name. This is just a fancy rehash of The Omega Man with speechless "Infected."
Also, Emma Thompson destroyed the world...but I can't be mad at her...she's Emma Thompson, for the love of Pete. I screamed when she appeared on screen for her little cameo. Too bad the movie wasn't worth her presence.
This film, the third in a series of increasingly bad adaptations of a brilliant novella, tells the story of soldier/scientist Robert Neville as he struggles to survive while searching for a cure to a virus that was intended as a cure for cancer. Of course, as such things do, it went horribly awry. Those who were injected with 'KV' mutated into sunlight sensitive cannibals with big rubbery mouths and no social structure (according to Neville.)
I already said that this is just a flashy remake of The Omega Man starring Charleton Heston. There's a considerable amound of tension and it's well directed, produced, and performed. The problem is that IT'S NOT I AM LEGEND.
Below this sentense, I'm going to spoil everything. You have been warned.
In the novella, Neville doesn't die a martyr...he doesn't save the human race. The vampires are the new world order and he's the monster murdering them in their sleep...that's the WHOLE POINT OF THE BOOK! That's the main thing every single adaptation of the novel has failed to grasp. The only version that has come close to the desperate isolation of the book is the first one: The Last Man on Earth starring Vincent Price.
Basically, if you've never read the book or seen the other films, you'll like it. If you have, you'll most likely find yourself very disappointed.