I never understood the point in customized 404 pages. I think the whole purpose is to tell the user the page doesn't exist, as quickly as possible. Since users are used to the standard page, the quickest way to do it is to show them exactly that. When I see a customized 404 page, first I'm fooled into thinking it's a valid page, then I feel disappointed, after I realize what it is.
The only thing I can think of is to create a way to try to get the user to the page they were requesting as quickly as possible. Or at least make sure you keep them on your site instead of them wandering off somewhere else.
