My 9 year old son read this book, and so I read it, too, so we could discuss it together. It's a very engaging story. My son read it in a day - he couldn't put it down, and I found that I couldn't, either.
There are some very good themes in this book -- the main character is very strong, determined, and survives despite having the odds stacked against her. One thing in particular that I liked is how she doesn't necessarily learn how to do new things with great speed -- but she keeps at it until she has learned a new skill, and is successful at it. I think this is a particularly good thing for children to read about. Not all skills are immediate, and working to achieve proficiency is worthwhile.
Other major themes are the importance of voting, the rights of women, the ability of girls to learn to do jobs formerly considered only fit for boys, facing fears in order to overcome subjugation and make a better life for oneself, and growing up without parents. Slavery is touched on.
This book lends itself to the discussion of any of these topics, and my son and I had some very good talks as a result.
I did feel the story ended a little abruptly. I was completely unprepared for it to be over, I was hoping more would be explained, and I felt dissatisfied after finishing the book.
I must say that I also felt a bit uncomfortable reading about the woman whom the story is actually based on. (She lived her life as a man, and it wasn't discovered until after her death that she was female.) While this book presents topics for children to discuss that are interesting and worthwhile, I don't know that all parents would feel that this book is appropriate for their children to read.
I would suggest that parents read the back of the book first, before handing it to the children to read. Be prepared for some questions!