I don't think it's an argument that they are kids book, course they are. They are about kids, they are written for the reading skills of kids, they were heavily marketed toward kids. Because of that, there are people who've never dealt with them that look down on adults reading it, say as anything that is developed for kids
However, they present a number of mature themes and concepts in mature ways, which as NFS says, is why they hold up when you grow up. If you'd never read them before, maybe you'd like them maybe you wouldn't, hard to say, but they clearly never appealed heavily to an adult demo during their release, and there are probably other books that do similar things better (wouldn't know myself: don't read).
In the end, if some one hates on you for enjoying something that you're not in the target audience for, they have too much time on their hands. If you still like 'em to hell with the rest, and if you realize that you've grown out of them, that's fine too.
