Except, previews being previews not everything is necessarily working. Just taking the preview I went to on 5/13 (I'm not going today) as an example:
- Curse of the Werewolf -- Closed
- Harry Potter and the Battle at the Ministry -- Closed
- Fyre Drill -- Intermittent. Down for a good portion of the day
- Mario Kart Bowser's Challenge - "Delayed" (i.e. it was basically closed)
- The Untrainable Dragon -- Toothless "was on an adventure" (i.e. The portion of Hiccup riding Toothless overhead did not occur while the background effects were still running. Felt odd). Also at the final reveal of the "untrainable" dragon, they had to reset. It didn't work. It was like a "Take 2".
So, if you didn't experience these things you might be inclined to return on opening day. I got a good feel for it and have plans to come back next March.