We did Park opening a couple times during Spring Break & boy I am glad we did! We got so much done the first 90 minutes or so, while the rest of the day was packed. We only stayed at the Park til noonish, then left for lunch, swimming, breaktime.... then back to the Parks to use some Fast Passes & stuff....
Off season, I didn't find it so necessary to get there early. But if I ever have to go during one of the school breaks again, I will definately be at the Parks (esp MK) 1/2 before opening.