Sure, we have been at both times:
Pros for May:
Weather is more predictable, not too hot, but definitely not cold... you should be comfortable in shorts no matter when you are outside.
moderate crowds ( we didn't wait longer than 20 minutes for anything)
Little rain
Flower festival in Epcot.
StarWars Festival in MGM.
Longer park hours
Cons for May: regular season prices on resorts
Pros for December (1st 2 weeks)
Smaller crowds
value season for the resorts
Christmas time events
Resorts and parks decorated for Christmas
Cons:
Unpredictable weather
Shorter park hours
Personally we would rather put up with larger crowds and more expensive resorts to have the warmer weather... so my favorite is May. I am sure not many will agree with me, though!