I really thought Will Smith did a very nice job playing the part of Robert Neville, and the writers, Mark Protosevich and Akiva Goldsman, developed the character quite well (I didn't like the ending though). The greatest difference with the movie character and Matheson's Neville is that the movie portrays him as a military doctor/scientist while the book has him as a factory worker. To me, it's more interesting to read about a layman investigating a disease that's wiped out humanity.
What really sets Matheson's I Am Legend apart from its movie counterpart is the development of the vampires. For one, the movie has them as just monsters of the dark that don't seem too intelligent. The book gives the vampires an identity, and makes them slightly sympathetic.
The movie is still worth watching, but I Am Legend is much better when read.