We just returned from our trip and our group of 6 loved both parks but particularly the HP attractions. Absolutely phenomenal! We had Express Pass and were so happy we did. It allowed us quick access to many rides.
None of the Harry Potter attractions though take Express Pass so each day there we headed for Forbidden Journey or Gringotts first thing. The lines and crowds definitely grew throughout the day.
We used Express on virtually all of our other rides though including Spiderman, Transformers, water rides, etc.
As mentioned, no extra charge for Hogwarts Express but you do need a park to park ticket and you have to actually exit the ride and board it again to go back the other direction. You can take it one way as a means of getting to the other park.
I have to say bag check at the entrance to the park could not have been easier or more efficient. So much faster than Disney.
We loved both parks. Have a great time.