I've finally gotten with the times and ditched CVS in favor of Mercurial for many of my active projects. I have to say, it's pretty nice.
One feature I really like is the ability to make local commits to a repository and then ship out all of the changes at once to other users (or to a central repository) in a single changeset.
Another cool feature is being able to rollback mistakes. And having an automatic repository wide id with each commit eliminates the need to tag all the time like in CVS.
This presentation at Google from a Mercurial developer is very good. Also, this talk from Linus on his revision control software, git, is good at explaining why you should upgrade if you are still using CVS or SVN. I wish I had switched a lot sooner.
Here's another good piece on git vs. Mercurial vs. SVN.