I had read this book and expected it to be very difficult to translate to the screen. I was pretty much right. They sped through the first third of the book that established the world that the movie was based in with a quickened narrative play by play. The biggest part of the story was that the "host" was still awake in the alien's new body and talked to her inside their shared head. The voiced over host communicating was lamely done and awkwardly over dramatic. The movie is still watchable but nowhere near as it could have been if they would have emphasized the sci-fi part of it instead of the fluffy lovey/dovey stuff.
I would lukewarmly recommend people checking this out. It's not a bad watch but you kind of are better off reading the book prior to seeing the movie for background knowledge. But the book is better and makes the movie somewhat disappointing.