We were there last March for Spring Break and it was great. The weather was gorgeous - around eighty and sunny every single day. It made me want to move to Florida! The place was busy but not crazy. We took good advantage of Fastpass. It is so easy to walk up to an attraction, get the little tickets, and go off to do something better than standing in a line until it's time to return. This way we waited for very little. My kids are NOT patient!
Folks here say that if you visit a park on an "Early Entry" morning, that park will get crowded by later in the day and you should "park hop" over to another. I also noticed that this March, the MK is open later some nights than others. I plan to go to the MK when it is open til 8, because on these nights they will have the SpectroMagic parade, which is glorious. It will also mean a chance to see Tinkerbell fly, right before the fireworks begin. We saw her last July and I sure want to see it again. I am terribly afraid of heights so the whole stunt REALLY impresses me! And keep in mind that the Animal Kingdom is only open 9-5 so you'll have to get up early if you want to go there.
See you at WDW!