It's obsolete and there is literally zero reason to use it ever again. More modern version control systems (e.g. git, hg, etc) provide a huge superset of the features that CVS offered and there are tools to migrate CVS repos. If, for some strange reason, you can't migrate a CVS repo you can probably still find a tool to use git locally (e.g. I know there's a git-svn tool that uses git locally and then commits to SVN. There's likely a git-cvs or hg-cvs tool out there somewhere).
The huge wins of modern VCS' are distributed repos (you don't need a centralized server), and vastly superior branching & diffing. There are a million other improvements, but these are the largest 2 IMO.