I went opening night for the past two movies. There were a few freaks in costumes but mostly normal people. When I say, normal, that refers to their dress, the general distribution tends to be geekier than normal.
If you are planning on going, you should probably have tickets already. Most theaters are already sold out. You should also plan on showing up at least an hour or two early. We generally drove past the theater around 9 to size up the line, then went to eat/drink. If the line was long we would grab fast food and a six pack and go back to the line. If the line was short, we would take our time at a sit down place and hang out at a bar for a little bit. Alcohol makes the people in the costumes more barable, but it also makes it more difficult to stay awake in dark theater.
As for me, this year Im going to wait. Some coworkers want to skip out of work for a few hours and go see the first morning showing. Cheaper tix and less crowds are worth the additional 10 hour wait.