We went over spring break with my 3yo autistic son. It went very well. We had the GAD fast passes 3 days which helped (6 rides in a row on toy story!). We stayed at the BTL so we were able to go to park in the am go back to the room for a break and go back to the same park in the even. I collected FP's all morning long so we could get on everything quickly in the evening. We did get and use the GAC, and it was so helpful. Chris does well generally, as long as we are moving, but to stand still in a line; he just can't do; we began to have meltdows. The shorter lines made life so much easier with him.
I will mention to remember noise levels in the movies. Some were a little too intense for Chris, next time we will bring ear plugs.
Our children really enjoy the charachter interaction, none of us enjoy lines; so if you can afford it I would recommended a few charachter breakfasts. We did them every morning and it was wonderful. The many of the charachters cued in really quick to Chris's special needs ( he looks perfectly normal, just that lack of eye contact and staring off) and spent a few extra moments with him to get him to interact. It is very nice and leisurely.
We also brought a little nano ipod. Which we kept several of his fav movies on ( and were able to tape magical moments) It was a lifesaver waiting in restaurants and lines. very small, I was able to carry it in my lanyard.
We also got a stroller pass GAC. We were not sure if we would need it, and we rarely used it, but when we needed it, it was nice to have. I used it at the soarin bldg, while everyone went on soarin; I was able to push him @ in circles in the building; otherwise he would have been running for the escalators and climbing the walls. Again standing still is not something he does well, but in his stroller he will often times remain calm, and not try to escape.
Hope you have a wonderful time, if you have any questions let me know