In the last years we’ve seen many SCM trying to replace SVN [1] (which in turn replaced CVS [2] in many places). Git [3], Mercurial [4] are two examples of that. I never really looked into them as they mostly sounded like “SVN with this” or “SVN with that”.
Then I discovered darcs [5] and I never went back. Sure, XCode has no integration with darcs, but one can live without that. What made me switch from SVN to darcs is the “theory of patches” [6] it is based on. It makes redundant most concepts like branches, revision and even tags are different (they are an empty patch depending on all patches that were in the repository already). This way it is really easy to branch (if XCode had any integration, it would make its snapshot feature incredibly powerful) and merge back. I also found it easy to handle conflicts (except on project.pbxproj files, be careful with those).
For Cocoa developers (like us) it is also really welcome that darcs puts only ONE directory (called _darcs) in the main project folder (unlike svn putting a .svn folder everywhere) so you are unlikely to mess things up, for example by copying nibs to localize them or things like that (which was plaguing me as PDFKey Pro is localized in many languages and I kept doing the same mistakes over and over with SVN).
Currently, a Darcs GUI for Mac OS X is in the works, is called Darcheology. [7] It is already quite neat but is in dire need of contributors, so if you got some spare time, it is surely worth to take a look at it!