A lot depends on the fantasmic schedule that night. My experience is it's busy for the lighting, lightens up during Fantasmic, gets heavy after Fantasmic for a bit, and then slowly gets quieter for the rest of the evening. Remember the lights stay on usually for a hour past posted park closing. The last half hour before they shut off the lights is my favorite. Lots of space to walk and you can get most of the photopass pictures with minimal wait. (That being said, I've never been there the last 10 days of December when I would guess all bets are off and it's constantly crowded)!