Any advantages to using a Custom 404 to rewrite vs. using an ISAPI filter?

Obviously, the necessity for elevated privileges when using/installing an ISAPI filter make the 404 approach tasty for anyone without admin access; but for those who have it, are there any other perks? Which is more taxing to the server and bandwidth?

bandwidth or more correctly data transfer, will be the same in both cases. The UA is not making extra requests to the server, the transfer between request and rewritten page is handled internally.

The question of extra server load is a moot point. With ISAPI filters EVERY request is sent through the filter, so this adds a minuscule extra load to every page request. With a 404 page, the script is only invoked if the requested URI does not exist.

I run half a dozen moderately busy sites with a 404 page creating a static looking URI with very little increase in CPU utilisation. And I run the server!