We've done this time period before with great luck. It can be crowded at the MK, but overall, the crowds are less than we've experienced later in the summer and much less than the Spring Break crowds. We like that time frame as it's warm enough for the water parks and resort pool, hopefully Star Wars Weekends have started by then and the crowds seem a "little" lighter.
We plan to be there next May at BLT the day after school is out. Unfortunately, the kids may miss a couple of baseball games, but we've sacrificed WDW for that the past several years for the heat and humidity of August.