This will be our 11th year at WDW in May. We always go either the first or second week of May. This is what we have experienced:
Weather: daytime upper 80's to lower 90's
It can be humid, not as bad as summer. Or it can be dry. We have experienced both! Last year it was cooler than what we expect in the evening, and that could have been, because it was more windy.
Crowds: We have seen it crowded, ( pre 9/11 ) and have seen it light. Now, I base that upon how long a wait is for certain rides.
Note: Only one year did we have rain showers just about every day ( approx. 5 pm) and that was 1999.
I hope this is of some help.