The enjoyment of this movie really depends on a few things. I love this movie, love, love, love, love, LOVE it but it is, hands down, the most depressing movies I have ever seen. There are several times during the movie where you will go "Aww" because the girl is so incredibly adorable but there are just as many times that will bring a tear to your eye if not a whole flood.
It's entertaining and educational and do not be put off by the sadness, this is a great movie and if you think you can sit through it, Grave of the Fireflies is one of the most memorable (animated) movies you will ever see.