I watched the movie before reading the manga. And it was good I did, because otherwise I would have thought the manga was pretty rushed. The movie was very loyal to the manga, but it changed some things, making them longer - like when Makoto is going back in time over and over again to save Kozuke's relationship with girl, or to save his life. We get to know more about Chiaki or her aunt in the movie.
It is the kind of story that makes the reader have a lot of unanswered questions at the end. Both versions are worth to check them out.