I think all the other posters hit it quite right. We have one more day, and it has been pretty crowded, but timing things to avoid crowds (getting there early, leaving from 12-2), scheduling our itinerary using a crowd calendar, using ideas from Unoffical Guide touring plans, and using the GAC made all the difference. It was like we were in a different park from the one we saw when the crowds were crushing in midday. I also think that early June won't be bad.
I did have moments when I felt a little guilty about using the GAC. My son has mental health/behavioral issues that we haven't quite diagnosed yet (many psych types are arguing about it), but he does have a provisional diagnosis and the psychiatrist wrote a note for it. When he was being charming with cast members in line, I was thinking, gawd, why am I using this? Is this fair? Then when I was sitting in a corner next to a bathroom holding him while he hit, kicked, and tried to bite me three hours later when it was time to go, I was thinking "okay, this is why." Everyone should plan their day and try to manage things that would be hard for the kids and etc., but when the stakes are extra high because of your kid's fragility, hey, I think it's okay. In the end, we would not have had this vacation if the GAC hadn't been an option, because I wouldn't have exposed him to the stress. I felt like with the GAC, there would be a better balance of good and bad stimulus, and in the end we all might have a good time. Despite two major meltdowns in the parks (both upon leaving; transitions are his biggest trigger), he has had a great week. All that is to say, use the GAC. Life is hard enough. We don't get much support when the issues aren't visible. Thank goodness Disney is willing to help.