Personally, I think this movie is overrated. I personally think that the 40 mins didn't fit the story. The charachters weren't depth properly and fall in love just because. The art is also a "copy and taste" of all modern animes, really poor. Also all the backgrounds were poor of details, the author could do a better work. But it had a lot of potential though. The plot is very good, and it's very different from the other shoujos. Also the music is just amazing.
Ill give it a 5 (mediocre), but remember that it's my own opinion.
I gave a 7 for the plot because it's a really good idea, perfect for shoujo animes, but the author didn't develop it properly and, at the end of the movie, I already got tired and it felt sickly sweet.
I gave a 4 fot the art because I don't like that kind of drawings
I gave a 8 for the music because piano felt so nice and the melody wasn't foregone
I gave a 2 for the charachter because they were flat and they, and also their relationship, didn't develop during the story.
I gave a 3 for the enjoyment because all the little things (and also the bigger ones) that I disliked didn't let me appreciate the beautiful story.
Hotarubi no mori e deserves a 5 because it could be much better but, after all, the story is nice and enjoyable.