IMO, I agree with those who said it's fun to do once. Beyond that, it's just a ton of money for something that isn't worth it to us. My kids enjoyed it a lot, but they would have enjoyed just being in the parks almost as much. None of us liked the trick-or-treating; we don't need/want a ton of candy. The crowds were very large so it didn't shorten the lines or anything. In fact, we used to go to DLR every October before they had the Halloween parties there. Now, we avoid October because of the crowds.
The first party we attended as a family was, unfortunately, on a very very rainy night. No one saw costumes because everyone had to wear rain ponchos. Trick-or-treat lines were changed and no one, including the CMs, could figure out where they'd been moved to. Rides were closed, parade cancelled, etc etc. It was kind of miserable trying to do the parks in costume in the rain. That was just an unfortunate circumstance of course, so we decided to try again another year. The second time, it was more crowded than any other visit I've made. We did have fun, but we couldn't get near most of the activities and my kids couldn't see the fireworks show. We had a good view of the parade, but it was a bit disappointing. We did very few rides, and mostly walked around admiring costumes (and being admired, which was a big deal to my kids).
We decided that it was a fun experience once, but the cost is just too high for the fun of walking around in a costume at DL. We may have run into bad luck between rain and crowds, but I'm not going to give it a third try. I *have* gone 3 other times with another single family member, but that was a long time ago when the cost was low and 2 of those parties were in DCA.
If you aren't paying for a (largish) family, I can see it might be a really fun thing to do over and over. If you've never done it, it would probably be a worthwhile expense. I'm glad we tried it but we won't do it again.
Have to throw in a picture of our family at the party, though! We were garden gnomes.