I was there in August a few years ago, and yes it was very very busy and there were long queues for all the popular rides. But the biggest problem was the rain - it rained almost every afternoon, and there was a tropical storm while we were there which meant constant heavy rain for three days. I would NEVER go back at this time of year again. People I know who have been in May generally seem to think that's the best time to go, both for weather and queues. While Christmas/New Year is very busy, apparently Easter is the busiest time of the whole year.
By the way we found Universal's Islands of Adventure and Universal Studios to be much better than Disney in terms of rides. As we were staying on International Drive it was really convenient for us, more so than Disney.