My family had a set of the Narnia books when I was a child, but I can't remember if they were ever read to me. Of course, I think I'm reasonably familiar with the books from the recent The Lion, the Witch, and the Wardrobe movie, as well as a few of the movies in the old BBC series. 
 Even so, The Magician's Nephew was completely outside of my cultural knowledge of Narnia. It was interesting to have such a "virgin" read, while at the same time knowing the stories that are being set up by the details. 
 C.S. Lewis was a Christian apologist, and it's pretty clear that he's not just writing a children's story. If I had to guess, based on the content of The Magician's Nephew, I would say that he is trying to a) introduce children to theological concepts, and b) write "correctly aligned" stories for children to read. 
 Unfortunately, there's a fine line between allegory and brow-beating, and I feel that Lewis crossed it. The condemnation of Uncle Andrew is strikingly anti-science (the guy even uses guinea pigs, for goobley's sake!), and the "we have to trust and obey Aslan no matter what, even if he isn't communicating his intentions to us" bit was rather much. 
 I imagine that a child probably wouldn't even pick up on these things, and there was plenty of really great ideas and excitement in the story. But reading it for the first time as an adult, it was rather groan-worthy at times - particularly towards the end. 
 Still, I felt that it was a nice way to prelude the rest of the stories, though I don't think that it would have worked as a stand-alone story. 
 All in all, I did find it to be an enjoyable read, and I am looking forward to getting to the rest of the series.