It was fantastic up until the vampires/zombies turned into cartoons. One of the scariest scenes I've ever seen was the bit where he goes in after the dog and the vamps are all huddled in a circle shivering in the dark. That was some top notch creepy. And it was brilliant acting by Smith (so much of the scariness of that scene was just because the character was scared).
But, then we got a closer look at them a bit later and they were flying through the air, bouncing off the walls, and just all around looking like cartoon monsters. I'm just not frightened by cartoons. And, it really killed the slow, calm, horror of the book for me. The things that made the book unique and scary in a way that sticks with you are just fundamentally different from the jump scares of the film.
The Vincent Price adaptation (The Last Man On Earth) remains the most faithful film version of I Am Legend, in spirit if not in specific detail. I don't really mind when a movie script changes some plot points or some character details to make it work on screen, I just dislike it when the spirit of the thing is thrown away and replaced with blockbuster tropes (which is my primary complaint about I, Robot, as well).
But, again, I thought Will Smith was excellent in the role. I just really hate what they did with the vamps.
