Inspired by the movie The Lion, the Witch and the Wardrobe, I read the Chronicles of Narnia for the second time in my life (I also read them during middle school), and I thought I'd offer a few thoughts, as well as a couple of links for further reading.
In my opinion, these books are very insightful, entertaining and easy to read. I read all the books within a two week period (granted, I didn't have too much homework during that time), and the conclusion of The Last Battle was better than I expected (don't worry, I won't spoil it—yet!). I have read other people's reviews and opinions of the books, and sometimes people say the books are simplistic or chauvinistic, but these allegations are simply not true. Anyone who says so is ignorant of the wonderful Christian tradition the books were written in.
The imagery with Aslan, Narnia, and our own world is simple enough for a child to understand, but it also contains enough material for much thought. I often put the book I was reading down to think about a theological aspect of the story or try to apply something one of the characters learns in the story to my own life. The conversations between Aslan and the characters are especially enlightening.
(WARNING: Possible Spoiler!) One question stuck in my mind as I finished the books, though; what happens to Susan? Obviously, she is not 'damned to Hell,' as some reviewers seem to think, because of her 'silliness' and infatuation with youth, but the reader is left to wonder what will happen to her after she finds that her family has died in a train accident. I, the optimist, hope she returns to her belief in Aslan and Narnia, and finds herself one day in Aslan's kingdom... but that's my opinion. I found two very good articles online concerning this very topic: Article 1, Article 2.
I encourage anyone with the least bit of interest in any kind of fantasy novels, or even in short, entertaining novels, to read these books. You will not be disappointed!