We have went in May for the last 2 years (first week) and love it! Great weather - not a single day of rain either week, 80's, not too hot sometimes breezy, DAK was the only HOT park both times but it's always the hottest it seems! We enjoyed cool nights where we were able to wear jeans if we chose and often a light sweat shirt was needed for EMH!
We changed our plans to go in Jan 2010, but I'm starting to rethink it based on my love for Disney in May, what I've heard of the Brazilian tour groups recently, and a few photos I just saw of the big crane taking down the lights on the castle during park hours the same week I was planning to go - THIS was a major bother when I saw it - so unattractive really! If anyone could verify that for me, it'd be great because I heard Jan. (last week is a great time to go and many said better than May!)
May has my vote but I was hoping to read Jan. myself!