I'm going to start out by saying that I am a huge critic of movies once I have read the book. Especially if the novel is fresh in my memory and I remember specific details. I freaked out during nearly every Harry Potter movie because they didn't follow important details from the books. I am a little obsessive about my books =)
I realllllly wanted to watch The Host, by the time I was ready to go to the movies and see it, it was out of theaters. That forced me to find it online so I could see it- I know I'm a bad girl, but I seriously couldn't wait any longer. I went into the movie with an open mind, really excited to see what they had done with this story.
I was a little upset that they changed the storyline of Wanderer and Melanie on the run from the beginning- it wasn't a huge deal but it did bug me that Melanie took over Wanderer's body and crashed the car. In the novel Melanie never had that kind of control over her own body- Wanderer always had the control to stop her from intervening at the beginning.
I was okay with all of that, until they started excluding characters. Characters were dying when they weren't supposed to be dying, characters who had a pretty big role didn't even exist. It was frustrating.
I was happy that the movie ended the same way as the book for the most part- except that Wanderer is now in a completely different body than was described in the book. I guess that's not a huge deal- but it did bother me because I'm weird like that.
Overall, the movie followed the general plotline of the novel but I would say that the book is a million times better. I will probably not be purchasing the movie to watch again in the future- but I would recommend seeing it, especially if you have read the book- it is definitely a great extension of the novel. Just be prepared that everything you read in the book will not be covered in the movie- and I suppose that's okay seeing as the book was so long =)