When we were in WDW in May of 2001, we were there the 3rd week of May, and it was in the middle of the big drought, so of course, we had no problems with rain... quite the opposite, we were all praying for rain, as there were fires off of 192, and actually one back by the Epcot Castmember parking area--- so it was very smoky.
As for crowds, they were moderate, and we had no problems seeing or doing whatever we pleased. The temps were in the mid 80's and the humidity was low.
May is probably my favorite time to go.. love the Flower Festival in Epcot, and the weather is just about perfect... warm days, warm nights, little rain, and lower humidity.