I'm a halloween freak, and my friend loves to dress up in costumes, so we pretty much loved it. Imagine the disney atmosphere, only 100 times more magical because EVERYBODY is dressed up. It's amazing. We had children ask to get their photos with us, there were photo ops set up all over, candy was handed out, characters who rarely show face were out--you're going to love it.
As for crowds, it's a little more crowded than Very Merry Christmas Party, but a lot less crowded than your typical day at the Magic Kingdom. Aside from Peter Pan, I don't think we waited longer than 10 minutes for any ride, and most of that was spent walking up and down the long empty queue lines.