I'll be at the parks in March and am preparing for big crowds...not huge, but big. We were there last March at the end of the month and there were a lot of people. But it's doable if you have a plan and are flexible.
I think the summer as DL will be crazy both this year and next with all the new stuff. It might be good to wait until next summer though, because then you'll get to see the new Star Wars, new parades, Little Mermaid (all these are opening for summer, 2011).