In 2001 my holiday spanned into Spring break. The first week there were hardly any crowds; the second week (spring break) the crowds were up one hell of a lot. For example, we queued up for 25 minutes for the Test Track at EPCOT in week one, in week 2 there was a 4 hour queue

(we didn't join it). However, if you manage the fast pass system efficiently things should be OK.
The only complaint I did have is that the American "chavs" seemed to think it ok to shout and scream in hotel corridors at all hours of the night and the number of people I stopped in the queues who were going to join their friends with walkie talkies at the front. They looked very miffed when I told them they had to wait like all the other decent folk!