I'll vouch for how good I Am Legend is...haven't read it in years, but I surely did like it....Omega Man was a good flick, but not good enough interpretation of it.
It's been a long time for me, as well. The main thing I remember is the central plot point, the reason for the title, that the hero has become the monster killing off the "normal" people who have found a way to deal with their new condition. He has become the legend that they will use in the future to scare their children. "Be careful or that evil man will come into your room while your sleeping and drive a stake through your heart."
It's been even longer since I've seen the Vincent Price version and I frankly can't remember if that plays into the end of that movie. I know it was completely ignored, if not turned on its head, in Omega Man. That movie is one of my favorites from the era, but it's only loosely based on the book.