We were there in April this year, the first week the resorts go from peak back to regular, so we feared the worst with crowds.
We were thrilled to be able to walk right onto all the rides in the mornings, and by using Fastpass throughout the day we never waited more then 10 or 15 minutes for anything, and most rides we walked right on with little wait.
Crowds were heavy at the parades of course, and wherever stollers were parked......lol
We ate at off peak times so we often had very quiet meals. Later in the day we would notice long lines at the major attractions and longer lines for food and snacks.
I think that April 18th is the start of regular season in 2005, I'd go any time after that and expect manageable crowds and much better weather than in the summer.
We didn't experience any rain at WDW and only one brief shower in two weeks.
We plan to be there the last two weeks of April again next year because it was so great this year.
Karen