I've heard great things about the book, so decided to check it out before seeing the Will Smith movie. Some thoughts:
1. It was written in 1954 and is set in 1976-1979. So the interval between writing and it's setting (22 years) is less than the time that's passed since it's setting (32 years, give or take). This makes for a few interesting anachronisms. The main character (Robert Neville) has to fiddle with the choke on his car, for instance. But on the whole, the book seemed remarkably contemporary given its age.
2. The book is set in LA and the movie appears to be in NY. No big deal.
3. I bought the version with Will Smith on the cover not realizing it was a collection of stories by Richard Matheson. So when the story ended on page 171, I still thought I had another 140 pages to go. I enjoyed the story a great deal, but I feel somehow cheated. I never got to brace myself for the ending.
4. The psychological elements seem really authentic to me: Loneliness, despair, violence, self-loathing. I'm not sure about the dog in the movie trailer - seems inconsistent with Neville' loneliness in the book.
5. Lastly, the "brilliant scientist" bit in the movie tagline better be misleading - Neville was an average guy in the book, and that seemed important to me.
A good read, as long as you realize it's short.