Having a standard 404 error page is better than having none at all, although a customized page is more preferred for several reasons. On the one hand, you can be sure that visitors receive an accurate HTTP status code: for example, if the requested content is no longer present on the site, this should be conveyed with the '410 Gone' message. The visitor then knows that this content has been permanently deleted.
On the other hand, you can provide a specially-designed error page containing related links (i.e. links to your homepage or subpages where the content overlaps that which the visitor originally requested). You could even add a search function for your website. By taking these extra measures and providing incentives, you might be able to prevent visitors from leaving your site straight after seeing the 404 code.
With a creative 404 message you may even find that visitors are more forgiving. Naturally they will be disappointed at not finding content they were promised, but an original or funny 404 page could make up for it. If done properly, error pages do have some potential.
Make sure that the design of the error message matches the style of your website and you already have the foundation for a good 404 error page. If you let visitors know in a funny and light-hearted way that your content isn’t available, you’ll hopefully get a smile out of them and they won’t hold a grudge. For inspiration, check out our article on cool and creative 404 pages.