The ending? A pure WTF moment. Am I right or am I right?
It goes without saying that the book was better. But if you're the sort of person who wants to know how the story should have ended without actually reading the book, there's always the option of watching the original movie made in 1964 starring Vincent Price.
And thanks to the wonders of the Internets, you can actually download it at the Internet Archive. Or if you're too lazy to do that, here it is, embedded just for you!
Now you can watch the ending as
The film though given the title of "The Last Man on Earth" and having had the protagonist's name changed from Robert Neville to Robert Morgan, keeps mostly to the plot found in the book and is so far the most faithful adaptation of the book yet.
I watched this the day before I went to watch I Am Legend and I thought it was a fine film. A bit slower paced but that's as it should be.