Went to the party last year, and my overall impression was that it wasn't anything special, but I was still glad we went.
Basically, the party offers Christmas themed shows, a parade, and fireworks that are not available before Christmas week. Then there are special meet and greets with a number of characters, and there are dance parties and cookies and coco. There are also special pins and merchandise for sale just for the party.
The number of people they let in is kept low enough that you will see a worst case scenario of about 10-15 minute waits for the most popular rides, with walk-ons to 5 minute waits being more common. If you are going during a busy time and just want to ride without the long waits, this is the way to go.
However, lines for characters, especially rare ones will be very long. I'm talking 2 or 3 hours long. I think there were even reports last year of longer lines than that. Staking a spot for the popular shows and the parade is about the same as well, with people lining up hours before the party even starts. There are a few last minute spots you can grab if you know where to look or don't mind an obstructed view, but if you want that guaranteed front and center parade or show view, expect to arrive early and wait. We did manage to see the midnight showing of the Tomorrow-land show no problem because by that time most of the park had emptied out, and we were able to get a last minute obstructed view of the parade as well.
It's really crazy. Once you get off away from Main Street and the Christmas shows, the park feels like a level 1-3 park and is fairly empty. But venture back over near Main Street or the stages, and it suddenly feels like a crowd level 10 day with wall to wall people you can barely fight your way through. This sort of creates a catch 22. If you stay away from the crowds, then you end up staying away from all the special Christmas parts of the party and the Christmas decorations. And if you really want to take part in the Christmas parts, you have no choice but to wade into the crowds and put up with the long waits.
The dance parties were the least crowded party event going on and very cute for little kids. If you are wanting your kids to get some great character interaction without waiting in those long lines, the dance party is the way to go.
We found the hot coco and cookies nice when the temp dropped to 45, and we found ourselves suddenly freezing.
Nothing can beat 360 degree fireworks, and there isn't really a bad spot to watch them. Although, I do prefer center of park near the carousel.
In my opinion, they didn't handle merchandise very well, as they sold out of pins and other popular items during the first few parties. I feel they should have allotted a certain amount per night, instead of selling everything the first few nights and leaving nothing available for all the rest of the parties. I was a little disappointed that I couldn't even get a simple pin or a shirt as they were all sold out and had been for awhile. Not sure if they will handle it the same way this year as last. I know there was a lot of complaining, so maybe they will make changes this year to the merchandising.
The decorations were underwhelming to me, and you can see them anytime. Yes, Main Street was beautiful. The problem was it was about the only place you could see any decorations at all, and Main Street stays so packed with people you can't see anything to begin with. The rest of the park had a few lights shaped like snowflakes splashed across buildings and a couple of bows or candy canes to mark the dance party or refreshment locations, and that was about it. I thought Sea World put Disney to shame when it came to decorating for Christmas.
Overall, we went to ride rides without a wait, and the party accomplished that for us. Purchasing a party ticket was also a much better deal than purchasing a 1 day ticket. However, if we were going to be going to MK during the regular day, I'm not sure I'd pay for a party ticket. I don't think the Christmas aspects alone would be enough for me, as the crowds are really more than I want to deal with in those areas, and I don't care enough about any character to wait more than a few minutes for their signature. I don't care how rare they are.
Rather or not you will find the party worth the money is going to depend on your motivation for going. Just have to have that rare characters signature and picture? Worth it. Really want to ride your favorite ride over and over again, no waiting? Worth it. It has always been your dream to see the parade live? Worth it.
Been at the park all day, and now need to fight the crowds for a spot to watch a few shows and the parade, not sure I'd spend the extra money, especially for a big family.
for the Xmas party, the decorations are there every day after thanksgiving so thats already there. The cookies and hot chocolate they give at the different spots is same at each spot and there's really only so much you can eat LOL. At least at the halloween party they give the kids fists full of candy at each spot and the CMs fill up the bag as quickly as possible with big handfuls so the parents can move on to something else 
