Personally I prefer the books, although movies 3 and 4 both brought forth certain elements that I really liked.
Though I think after book 4, Harry Potter kind of lost it's charm for me. Somehow, after 4, it started getting more melodramatic and dark. I thought I'd like it if HP was a little darker, but it turns out "dark" storytelling really wasn't why I started reading HP in the first place.
I still read them, of course, but I haven't been very happy with the books that have come out after Goblet of Fire. They're better-written, but they somehow have less heart.
Ehm... to me, anyway, I'm sure a lot of people out there loved them. Just not me.