Looser adaptations are nearly ALWAYS the way to go on sci-fi books, especially ones more than 20 years old. By that I mean, they need to update the setting and often staple more realistic (less Vulcan and sexist) personalities on the characters.

The problem comes when they miss the core concepts of certain characters and whatever the "big idea" in the story is. See: I Am Legend.

What sucks about "I am Legend" is that they made the good movie and went back to deliberately shit it up because the ending was a little sad according to focus groups.