Sometimes unpreditable...but definitely better than summertime in the long run.
We've been to WDW the last 2 Decembers. In 2001 it was warm and mild the whole time and not a drop of rain.
Last December it rained all night the first night and turned quite chilly the next day with drizzle all day (I had to buy a Minnie Santa hat to keep my ears warm that day!). The days that followed were gorgeous.
When we get rain in the winter it's usually due to the passage of a cold front, sometimes they stall and cause the rain or sometimes they sweep through and we may even have storms that may last a few hours or sometimes the temp may just drop without the rain. Depends on the jet stream that time of year. When it does get cooler, if the sun is out it's usually comfortable, but it can get chilly if the sun is not out.