I've been about 5 times now (do tourist stuff when friends and family visit me) but I will NEVER forget the last time I went last summer. It one of the worst experiences I ever had - and I just want to warn anyone who is thinking of visiting when it is hot out. During summer months you can expect a tons of people to visit the building daily. The line is very long to the top and they herd people into long lines inside the buildings upper floors. The hallways on those floors are very narrow, packed with people and VERY hot and stuffy. This coupled with a long wait is VERY uncomfortable. They have a couple small fans in the corners of the halls but it does nothing. When I was there last summer they actually pulled pregnant women and the elderly out because it was so hot and unbearable.
Point is, if you are going to the Empre State Building this summer, get there when it opens or go right before it closes. Or at a time that will minimize the wait because there is no air in that building and wait is long and unbearable in the heat!