I picked this book up while I was at half-priced books, it was one of the staff recommended books and it had SCARAY vampires on the front and a nod from Brian Lumley one of my favorite horror writers thanks to 'the necroscope' run.
I think what it nailed best, and what most post-apocalyptic books tend to miss, is what isolation can do to a man. The vampire stuff is neat, but what becomes more and more clear is that the notion of vampires is simply something familiar for the reader latch on to as the book deals with a much larger concept. The ending was fantastic, a great end to a conclusion that brought a sudden and fitting end to the calm the character finally finds in his isolation. I Am Legend has a clever use of perspective that really keeps the reader from falling into any place that's comfortable and this alone is responsible for the many thrills this book offers. Its subject material makes it a horror story, but its pulled off as a very intelligent thriller, or maybe even a drama.
The current edition of I Am Legend also comes with some of his other stories which I am still going to read through, but I never thought The Necroscope would be replaced as my favorite 'Vampire' story.