I've visited Harry Potter World twice now, once in the middle of summer, the other right before Christmas. I loved both visits, but I think the Winter visit was better for a few reasons.
The HP stories seem more magical in the winter, there is snow on the castle model, and night falls quickly so walking outside with hot drinks is amazing. Not to mention it was also nearly Christmas, which means there is a general level of excitement as well.
My favorite bits had to be walking into the main hall, the huge clock, the glimpse into Dumbledore's office and the Gryffindor rooms, and then outside on the bridge, and Privet Drive. I loved seeing all the behind the scenes work as well, and don't worry, none of the magic is spoiled.
Highly recommended and you can see all my photos on my Harry Potter Pinterest Board HERE.