I haven't read the book, but here's my review.
It was a good premise for a movie (plague wipes out humanity), and the desolate NYC setting was great. The plot wasn't bulletproof, but it was interesting and at times intense - Like in the previews you see him talking to mannequins, which is funny at first, but there is one scene in particular when you can really feel the desperation Robert Neville has been living with since everyone died. The downfall of the movie is that the zombies were just too over the top. They were like 28 days later... on crack.. with superhuman strength. It wasn't believable. The best parts of the movie were the parts before and after encounters with the "dark-seekers". I left the theater wanting more depth. It was just a short movie - they set it all up and get you into it, and then it ends rather abruptly.
I give it 3 out of 5. Nothing I'd recommend a special trip to the theater for, but worth renting for a look.