I watched the movie and then read the book after and I actually like both for different reasons.
I liked the movie because other than the typical ending (only way to describe it) it was actually a pretty good movie. The first half of the movie with Will Smith being alone was awesome and there were some awesome mood/tense scenes (when Will Smith was looking for his dog the first time it ran into the dark area). The ending wasn't that great but it also wasn't bad. All in all, a pretty good movie and a lot better than a lot of the crap out there.
The book was pretty good too. I really liked how it tried to explain why the vampires were afraid of mirrors and crosses or why they had to inhale garlic vapors. I don't think I liked the beginning of the book nearly as much as the last half, but still a great little story. The ending was great, though I already knew it because I was talking to buddies after the movie and they told me the book ending.
This doesn't have anything to do with I Am Legend, but I also read Matheson's other short stories and I actually wasn't a fan. In general, I just don't like his writing and the short stories were just meh. 30 years ago when they were written they might have been more interesting, but most of them just weren't that good.
As for comparing the film and book, I think the first half of the movie did pretty decently and even did pretty well up towards the ending. Sure, you're missing the explanations, the girl being a vampire, the dog is completely different, and they added a kid, but they're pretty alike in spirit. I don't think either did a good job of telling the events leading up to the main character being alone (especially the movie) but it was a nice little bit of mystery.
As for Ben Cortman, there was a Ben character in the movie, but they wanted the vampires in the movie to be more animal like (notice that when they bite and drag someone off they do it like a lion which is actually silly because lions or large cats bite like they do because of the specific way their teeth are shaped) and they wanted it to be surprising that the infected could make traps. Ben was the main vampire that smashed the glass and he was alright, though I would have liked more of him.
The ending of the movie was just bad compared to the book and it's a shame they didn't keep the book ending but they really couldn't with how the movie was done. I actually did hear that they had to reshoot the ending of the movie so hopefully when it comes to DVD they will include the original and it will be closer to the book.
All in all- both are good.
