Perfect weather, no worrying about rain or cool weather, but more crowded than you'll generally find in the winter. Temps run in the high 70s to mid eighties. Quite pleasant.
Crowds are higher but parks are open later. We've had weather between exceptional and cloudy...think it's called the "June Gloom" out there.
We've been to DL in January the past two trips and am not sure if we'll go back again soon during this time- the amount of closed attractions during the winter (and especially with all of the construction) has been disappointing.