I'm a HUGE Narnia fan, I have been since I was a child, and I think C.S. Lewis is awesome, and indeed he was a fellow of my College so I won't have a word said against him. He and Tolkien used to sit in pubs in Oxford when Lewis was there (he later moved to Cambridge of course), and so you see each other's influence in each of their worlds, however, I must bow down and admit that Tolkien's world is so much richer and more complex that I don't imagine that the movie will be as big as LOTR.
However I still think it will be a big success, and a stunning film, especially with the rich interpretation of WETA. I suppose the big hurdle for Narnia is that the main characters are children... I don't know why but I think that makes it much harder on-screen. However, LOTR may have paved the way for Narnia to some extent, drawing in entirely new people to Fantasy - people who wouldn't have imagined themselves to like such things.
Just my tuppence worth

What does everyone else think?