We are going May 10 through May 15. There's no telling what the crowds will be with the celebration kicking of the week before. I'm not too worried, though...it may be higher than a typical May, but I don't think it'll be like the summer months.
Even if it is jam-packed, there are ways to maximize your time in the park. You can get to the parks in the morning when they open and use fastpass.
I have heard the weather in May is hot but not unbearable...maybe in the 80's...