I didn't read this book until I decided to teach it. I taught it to seventh graders. Although it is put at a 5th grade reading level, the depth of the implied themes in this book and the chunky descriptions lend it more towards 6th or 7th.
Anyway, Meg was a strong young female character and the strong ties of family and responsibility are infused with her. It has been a long time since I have read this book but I remember feeling like students would like this better if they could grasp images and analysis of Meg's situation and draw parallels to the real world.
Someone said on his/her review that the presence of evil was strong in this novel. I'd like to liken it to evil on a level between Narnia and Oz, but cryptically tagged with "the road to hell is paved with good intentions", etc.
I shall reread this one and then write a deeper review.