We went last Friday as the end of our four day DL trip and had a blast. This was our first time.
We booked our trip without knowing about the party, then decided to go but really wondered if we'd like it. My DH was especially skeptical. We had a wonderful time! DD2 just loved it!! She was dressed as Minnie and had so much fun meeting the characters. We did some trick or treating, some rides, the late parade and the fireworks. DD had taken a 4 hour nap, so she went until midnight, which really surprised us. But she was pleasant and happy and having a blast, so we kept going.
The lines for lots of rides were quite long, as were the lines to meet the characters. We waited 45 min to meet Mickey and Minnie. My DD had met them several times already on this trip, but not together and not in costume. She wanted to wait and was a real trooper- no fussing at all.
It was crowded, but only in certain areas. It certainly wasn't the worst I've ever seen it.
Our single annoyance was getting into the parks. It was madness! We waited forever at the first gate, then had to wait again to get our bracelets, then had to wait again for a bag. They had a parade going on when we went in, and all the people showing up for the party were really stuck because of the parade. People were quite rude- pushing and shoving. My DH put my DD up on his shoulders as we were quite worried about the crush of people, and no one seemed to care about her in the stroller- people were pushing past us as if we weren't even there and one person almost fell into the stroller! That was our only experience with rudeness, however, and the remainder of the night was pleasant.
I would go again if those dates work out again for us, but I wouldn't plan a special trip around it. We did have a good time, though- better than I anticipated.