First of all, I wouldn't say that I'm a HP fan. Of course I like the books, and I've reread them a lot of times, and I watched all the films except the 4th (the disc with it that my mother's friend gave us had so low quality that we weren't able to see anything and then we gave up, and it somehow didn't matter to us because my mother is not a fan and I've already read the book). People who are fans of something usually (in my opinion) like the characters or the plot of the books, while I like the world itself the most. I don't know many details and might forget a lot. I don't even have favourite characters and I never cared about them much, maybe because I was too little. I would certainly feel different about them if I read the books now for the first time. I also wasn't very observant. For a while I thought that Draco's father's name was Lucilius and that Ravenclaw's color was brown, and these are just a few examples.
At the time when the second book was published, I was 9. Everyone around me loved these books and was saying how good they were. Being a nonconformist, I immediately thought that something loved by everyone and that popular can't be good, and decided that I would never read HP. But then my cousin (who is only a year older than me but was a huge role model to me then) sent me the 2nd book, saying that it was very interesting. So I've started reading from the 2nd, and then he sent to me the 1st and continued with all the others as soon as they were published (he read them and then gave them to me). I thought that everything that he recommended was worth reading, so I came to the world of Harry Potter.
And... I can't say that I ever really left it. Moreover, I went farther and farther in, discovering forums and other places in the Internet, finding new friends and hobbies. There was a forum connected to HP that helped me in my darkest moments, and I'm still going there even though 10 years passed since I first came there. My friend and I had a tradition to go to the cinema to watch every HP movie that was released, and it was one of the things that was making us go meet each other. I watched the last HP movie when I was 19 and went to visit my friends (most of which I met on HP forums), and when it ended, they were crying and saying that it's over, our childhood was over, we were grown-ups already and the big era of our lives ended then and there. I didn't feel that. Surely the childhood wasn't over then or even now when I'm 25.
But honestly, it was and still is a huge part of my life. A part so significant and important that I usually don't even know that it's there, thinking of it not like "HP in my life" but more like my life itself.
Will HP still be appreciated in 20 years? Of course it will, 20 years isn't a big deal. It's more interesting whether or not it becomes classic fantasy. And I think it will. Professors in my university told us that Harry Potter is a good book in terms of language, and that it's very educational and all... I've heard the recommendations to read it in the original many times (I didn't because I've already read it, but maybe one day I will). Harry Potter isn't just a book for children, it's a phenomenon that started a new thing in the fantasy world and in the lives of many people, and I think it won't disappear any time soon.