We went in 2007 and 2008 and absolutely loved it and we're going again this year.
I am a Christmas fanatic, so it is well worth it for me to see the Holiday Wishes, parade, and the sights/sounds of Christmas at MK. We don't travel late enough in the month to see those things for "free" (after a certain date, they do the Holiday Wishes and parade for all guests, not just the party guests).
We don't use the party for extra ride time, although we will go on rides if the lines are short. We use it to hang out, watch the shows (even though they got rid of my favorite one), parade, fireworks, and just take it all in. That is well worth the price of admission, imo.