There are two Harry Potter sections, one in each of the parks, and they are both amazing. Diagon Alley is at Universal Studios and Hogsmeade is at Islands of Adventure. We took the Hogwarts Express to travel between them once, and that was neat too. I wish we had watched the movies before going to the parks because no one in my house has been interested in reading the books yet (I read the first two or three a long time ago, but no one else has), and it has been quite some time since we last saw the movies. It was all still pretty fantastic though.
Harry Potter and the Escape from Gringotts was my favorite ride.
We all enjoyed the Potter areas, and if any of my children become fans of the books, I would love to go back for an even better experience. There were a lot of children dressed in Potter type robes carrying wands (there were areas where you could stand on a spot and perform a spell to make something move) and a lot of unique individuals (you know, piercings and brightly colored hair) in Potter world, and that was all well and good, but I did think the adults dressed in costume with their own wands in hand were a bit odd. Whatever you like, I guess, but I can't imagine spending $50 or so on a "magic wand" for myself.