We've had great luck the first week of March - it started to fill up with Spring Breakers just as we were leaving. The bad news is that it is considered peak season - so resort rates are higher, but the good news is that the crowds are pretty low!
The parks are not open very late - Epcot is the latest at 9 pm usually, but as crowds are so low, and the weather is awesome - you can get most everything done in the shorter hours! We just got back from our first summer trip (June 25th - July 2). I couldn't believe how little we were able to get done - the crowds and the heat really wear you out! Thank heavens for e-night - we'd never done that before - but it is worth the extra money!
There is only one water park open usually - the other doesn't open till the end of March - beginning of April I believe. Usually, it has been Typhoon Lagoon, but this year they switched the rehabs around, so Blizzard Beach will be open in early 2004. The value resorts are always pretty full - but they are prepared for that - so have a ball!