I use gmail, and the spam marking feature, and the fact that it's so easy to use, is very nice.
Every once in a while I see vacation messages posted to mailing lists. Every single one of those I mark as spam in gmail. Partly I do that because people who don't know enough to set selective filters on their vacation messages are too dumb to listen to.
The gmail filter will learn from vacation messages which words score high as spam and perhaps future vacation messages will be marked spam and I'll see less of them. Also, the authors may start to score higher as spammers. That's a good thing too, for me. I'll see less of their mail since their mail will automatically go to the spam mailbox, and when I go in there to confirm which emails are spam, I get a chance to despam those emails which are important.
I don't think I've seen gmail do that yet though (filter mainly on the sender's email address), I've seen Bob Reyes' spams about his hosting service end up in the spam mailbox, but that's just because the email was spam, not because the emails were from him.