We've gone Halloween night twice now; last year and in 2002. I figure if I'm going to WDW for Halloween, I might as well be at MK for it!
It is crowded but I still had a great time and would do it again. If you go in with a plan and priorities set, you will do fine. Also, the crowds all depend on where you are in the park. You can ask one person who hung out in Fantasyland and they'll say it was packed then you ask someone who hung out in Adventureland and they'll say it wasn't bad at all. YMMV.
The trick or treat lines seem long but go very fast. Get the photo done right around 7:00p when you are fresh and the lines are short OR wait till later in the night when the lines die down.
I LOVE the parade. I guess it's a matter of opinion but we loved the parade; the Headless Horseman is really neat!
Good luck deciding!