We did last last summer in July. It was HOT, but they have fans all over, so it was okay. While sitting and eating the heat was fine. When we were up dancing it did get hot. We are going again next week, it was the one meal my kids wanted to do again!! The food isn't bad and the character interaction just can't be beat.
When we were there, no one really left early. I think we all left when it was over, as everyone seemed to be having so much fun. However, I think the actual meal is done by 8:30 and the characters do come out pretty early on.