Our two previous trips we've arrived just in time to catch the last party on the last Friday night. We've loved it.
A few things I think contribute to this:
In 2002 we went in early, ate at LTT, and rode the Tomorrowland attractions until the party "opened." In 2000, the only people in the park early were at the restaurants (we were at CP) and had to stay there until opening time.
In both cases, we went right to Fantasyland and had about an hour in which there were no lines anywhere. The front of the park is probably a madhouse, with lots coming in and holding first parade spots. But we didn't experience any of that.
Our longest wait times the whole evening (in both trips) was around a ten minute wait for Splash Mountain and Haunted mansion.
The first year we went to the Belle show about a half hour early, sat on the first row, and kids were asked to be part of the show (they declined -- the weenies) but DH played the Beast.
Around 10:00 we make our way to Main Street and get a spot for the second parade. Then we take turns shopping the candy store and Emporium until show time. The second year DD helped warm up the parade crowd with a CM while we waited by singing Christmas songs.
I think one of the factors is the importance of shows to your group. If we were trying to fit in a lot of those, we would be moving with big crowds in limited time frames, which would eliminate a lot of ride time that we enjoy.
All that said, we are getting there early enough to attend the Thursday on the 16th this year, so I will be interested to see if there is a discernable difference in attendance.