For my personal development, I currently use CVS. Yes, it's a bit dated, and Subversion is generally intended as a replacement. However, it has a number of what I would consider basic, but serious issues that are still holding me back - and I would encourage others to contemplate these as well before making the switch.
The issue that stands out to me the most is the inability to remove versions or files - a trivial operation with CVS or just about any other version control system. See http://subversion.tigris.org./issues/show_bug.cgi?id=516 for the details and some rather involved discussion - even talk of a bounty to get the issue resolved!
For the most part, the idea of source/version control is the ability to go back to practically any point in time. However, sometimes there are compelling reasons for removals. What if a large amount of information, such as an entire hard drive, or confidential information, such as a password, is accidentally checked-in? The only remedy at the moment is to dump/filter/load the entire repository.
Wikipedia also has a few other comments on this.
Benny Bottema also posted a very good article on this at his blog, as shown in his comment below. Linking to it here for convenience: http://blogs.quintor.nl/bbottema/2008/03/01/subversion-obliterate-the-forgotten-feature/