we just did Crystal Palace at 8:05 on May 22, and Hollywood & Vine at 8:00 on May 23...parks opened at 9:00. You are in a nearly empty park, so can get some good pictures, but you don't get to go line up for any rides.
In the MK, after breakfast, we were allowed to walk through the castle to the entrance to Fantasyland...there was a rope there, but was really nice to already be at Fantasyland when the park opens...we just walked around the corner & walked on Peter Pan...then walked on it again 5 minutes later when DH caught up with us (he'd gone to get Splash Mtn fast passes.) Crystal Palace breakfast was fabulous.
At DHS, the service was really slow at H & V, so even though we were seated shortly after 8, we didn't get out until almost 9:00...at which time there were huge crowds at the ropes so we really didn't get any advantage from being in the park early for breakfast. H&V breakfast was not nearly as good as Crystal Palace, either.