In Mail.app, I've created one very important smart folder: "unread". This way, I have one place to look at for new mail, and the filters still sort them to the right mail boxes. Without this smart folder, I've had to go to every mailbox that has got new mails.
In the Finder, I don't use them. I'm smart enough to file my files to a good folder structure. I know Apple and Microsoft and Google etc. want us to file our files _anywhere_ or just drop them in one big folder and to use search and smart folders to retrieve them, but people who actually do this... I dunno... They're creepy.