First, let me just say I love this novella. I am a huge fan of movies and literature. In fact, I am a teacher and I have always been disappointed that I didn't have a watchable version of this movie that followed the story. They always make it into a monster movie or horror story and it's not either of those.
Without giving anything away, the entire book is basically in this movie. There are some extras but they make sense. There is nothing gratuitous in this movie that doesn't fit in context with the story.
Guillermo Del Toro may get around to adapting the musical but that isn't the story Stevenson wrote at all - this is. I bought the teacher's edition of this on Blu-Ray which had a study guide and screenplay with it.
I have shown this to my middle school students and they really enjoyed it after reading the novella.
Sure the movie is a little low-budget but it's ten times better than any of the educational movies I watched as a kid.