We have entered the parks for breakfast before park opening and you do not have to leave the park and come back when you're done eating. As the previous poster said, the rest of the park is roped off, so you can't go into the park past the roped off area. However, you do have a headstart over the regular park visitors. For example, if you have an 8 AM ressie at Crystal Palace, once you're done eating, you can wait at the rope at the land of your choice. So, while everyone else is back at the train station waiting for park opening, you're already right there at Adventureland, or FantasyLand, or whichever section you want to visit first. So when the park opens and everyone else is going under the train station and up Main Street, you're already getting onto Dumbo, or POTC, or whatever ride you want to ride first.