I think the Potter books went off the boil after number 4, they got longer and bogged down in lots of details and I thought book 5 was quite repetitive. I was bored by book 6 and still haven't read book 7 - or maybe I just outgrew them? Whatever anyone thinks of the merits of the books, and of course, given the discussion above, everyone's entitled to their opinion, they've still been a publishing phenomenon which was, I think, a mixture of luck that it first become successful and then it all just snowballed, and once the movies started being made, there was, of course, no avoiding them!
As David said, no one can predict what will become successful in film, books, TV series, and as many others have said, if they knew the secret of success they'd bottle it. I agree they didn't start to become popular until the 3rd book, but they seem to have got a lot of children (and adults) reading, and if those readers then move on to something, shall I say perhaps more sophisticated, then I don't see what's wrong, especially if they instil a love of reading into people who might otherwise not pick up a book.