I read HP for the first time earlier this year, after years of scoffing at it. I had tried to read the first book before, but could never get into it.
A friend of mine suggested that I persevere, get through the first couple of books and he bet me that by book three I would be hooked. He was totally right.
I really struggled with the first couple, they do really seem like mediocre childrens' books, but the later books are so much better and so much more adult.
I read all seven books in the space of a month and immediately went back and re-read them ( skipping the first two! ) but you have to read the first two to get the basis for the story and the characters.
I love how recurring themes and characters and incidents crop up throughout the whole seven books. Things that happened in book one are still relevent in book seven, even very small things. Very clever.
I will re-read the books again and again, but I do think the films are bad. You can't base your opinion of HP on the films, and you can't base your opinion of HP on the first two books.
I was a hardened HP sceptic, but I'm now utterly converted.