We've done the Halloween party about the past 10 years. Many of the times, it was sold out that night.
I agree with the others about the treat lines - they move very quickly, so you are not waiting. It's basically get into line and walk very quickly to where the treats are bring handed out, get into next line and repeat.
For attractions, the waits are generally very short. Basically getting in line and walking pretty much as fast as you can to board, so
DAS is not used for most because the wait is going to be 20 minutes or less.
The exception I have heard is the 7 Dwarfs Mine Train, which does tend to have long waits even during the party.
If you want to ride something that has a posted wait of over 20 minutes, it doesn't hurt to ask. But, the times we've been, even Space Mountain, Splash Mountain and Big Thunder Railroad have not had long waits.
The shortest waits will be during the parades and later in the evening.
As the others mentioned, the second parade is less crowded. The handicapped viewing areas are generally VERY crowded for the parades - both parades. They do line wheelchairs/mobility devices across the front of the area and the rest of the party kind of smooshes in behind.
I agree with the other posters that you would do better to avoid them and pick somewhere to watch from. The castle and Main Street area tend to be busiest, so avoid them if possible.