I liked the first 3 books or so, but after the 4th, I was kinda torn. On the one hand, they were fun easy reads. However, it seemed like part of Rowling wanted to take it to deeper areas, but on the other hand she wanted to keep it as the same formulaic episodic progression, i.e. Harry's at his foster parents, not happy, he goes off to school, goes through some misadventures, comes home. At the end of the 4th book, I got really excited because I was like "wow, this overall plot is starting to go somewhere" and then the 5th one was just the same thing and I realized that nothing is really gonna happen until the last book. Yeah, a character bit the dust, but there was a large amount of material in the Order of the Phoenix which just seemed like padded filler to me.
And I do realize they're fantasy books geared towards younger readers, but so were the Narnia, and Dark is Rising books.