Last year, we went from about March 11-17 and it was very crowded. However, the weather was perfect and the parks were open late. By following the touring plans in the unofficial guide we didn't really have any problems seeing everything we wanted. We would just get to the parks before opening and do all the really popular rides then. Then, we utilized fastpass throughout the day and usually took a break and went back to the Poly for a few hours during the really large mid-day crowds.
Have a great trip!