Do not watch the movie.
Here's some reason to that suggestion, Blake:
The movie's animation is far below the quality of the anime.
The anime slowly put together the pieces that were in the Visual Novel (I'm still angry they cut Hiiragi Kappei) while the movie tries to do it all in one go.
Due to the problem of trying to sum up the story of Clannad into an hour length movie, many characters are cut off and subsequently, so are their storylines.
AND THE FINAL REASON:
NAGISA ISN'T ALIVE IN THE END IN THE MOVIE.
