I did a walk in yesterday (6/30). The line began to form a little after 10 am for 11 am opening. They created 2 lines: one for upstairs, and the other for downstairs. Upstairs you can only get the appetizers and drinks. Downstairs you have access to the whole menu. People with reservations are seated first, with the first one beginning at 11 am. They began taking our names down at about 10:50. I was the 2nd party in line, so I was seated at 11:05. Since I was solo, I decided to sit upstairs so I wasn't taking up space for those with people, plus I had sat down there before already.
When I went on opening day with a reservation, they gave my friend and I the option for upstairs or downstairs, so we chose downstairs. That was about a 10 minute wait. In my opinion, downstairs is better than up. You have a larger variety of food choices, and you really see the pixar decorations down there.
Walk ins are doable, but the shortest wait seemed to be earlier on. They are also getting more organized with the lines. Opening day it was a little confusing. When I was sitting at the bar area, the line wrapped around, but I noticed it quickly dwindled down. Plus if you go early, you know for a fact that there are tables available, rather than later when people are sitting for a while.