People who haven't read the book don't have anything to compare it to; however, those who have read the book might find the movie lacking in details that made people not want to put the book down.
There might be more, but here are the biggest reasons that "Safe Haven" the book is far superior to "Safe Haven" the movie.
In the book, we see Katie planning out her escape from her abusive husband. We learn how she has saved money and even packed a suitcase. However, in the first scene in the movie we see Katie hurrying in the bus station and walking between buses to escape. We don't find out until later why she is running away or even who she is running away from.
In the movies, we are only told much later through flashbacks while she left. However, in the book we do see the couple together before her escape. We learn about her husband at the very beginning of the book.
We don't even know that Kevin is Katie's husband until halfway through the movie. Around the midpoint, we finally learn that Kevin isn't an ordinary detective on the hunt for a fugitive, but the husband Katie has escaped from. We might think Kevin is a good policeman until he falsely lists his wife as a murderer in a national database and the poster ends up in the town where she has found her "safe haven." By the way, that was not in the book at all.
There's no actual suspense about how Kevin finds Katie in the movies. There was a lot of mystery in the book and Kevin's search was more intense. He didn't use the database to track her down because he was leaving his job every change he could to track her down himself.
The character of Jo is not typical of Sparks' style. However, she was in the book and in the movie. At least in the book you see Jo as Katie's confidant. In the movie Jo pops in and out of the story basically whenever it's convenient, giving Katie someone to walk with and talk to. Until the end of the book and the movie, we find out who the mysterious woman really is.
If you do read the book, pay special attention to how Kevin found out where Katie was. It was left out of the movie entirely and replaced with Kevin breaking into a neighbor's house and listening to a message on the answering machine. The book definitely has a more interesting of Kevin finding out where Katie was.
Since it takes little time to read the book, you might be better off spending a few hours reading the book instead of seeing the movie.
So far, "Safe Haven" the movie has not gotten good reviews. Now we know why.
One reviewer wrote:
"Safe Haven" gets the romance right . . . but everything else so, so, wrong. The ending of the film is so jaw-dropping terrible that if you see this, you might, quite possibly, leave the theater angry that even Sparks thinks that you’re that dumb. The filmmakers should have cut this ending out if they wanted to salvage some of their dignity."