Last Man was much truer to the book, but skipped over some of what I found to be the most key parts of the book. And the ending... the ending of I am Legend was the culmination of the entire point of the book as I saw it. It had two key elements. Spoiler in white below:
He never cured the disease in fact he noticed at the end that it wouldn't be cured and in fact had mutated so EVERYONE was going to die anyway.
And he was formally executed because he was legend, not hunted and impaled on a spear because the woman couldn't tell them that his blood was magical and would save them all in time to stop them.
Both of those were COMPLETELY flipped. To the point where it's like... ok, so what's the point of this now? It was just a vehicle for Price at that point. But it was kind of what I expected from the Smith movie... pretty decent up till the end, then as soon as the girl shows up, they drop the ball.