we did lunch on our first day there. They started letting us form a line for lunch early, about 9am. They had rope lines set up. Then once the breakfast crowds got thru, they let us wait in the rope lines inside. They started serving lunch about 10:30 I think.
No reservations needed, but if you aren't in line by 11:00, then expect to wait a good hour to eat.
The food was good IMO. And keep an eye out for the shadows of house elves cleaning and owls flying into the windows.