We went this past May from the 22nd to the 29th, and boy was I surprised with the crowds!!
Our last visit was May 2-9th, and it was the most perfect time, great warm weather, but low to mod. crowds.
I'd never go again in Late May, it was just too crowded.
Once the kids from down South, especially Florida, get out of school, (around the 15-20th of May), they converge upon Disney. I know this because I talked to alot of people at the hotel. They got Florida room only discounts at 50% off rack rate.
I'd go in February or early May. Also, I'd avoid President's week, my friend went during that week, and said it was also very crowded!