Which SCM system for Xcode?

Xcode only supports Subversion, Perforce, and CVS. However, there are also distributed version control systems out there, such as Mercurial, Bazaar, and Git. These have no Mac-native GUIs, but you should still consider them. Personally, I love managing my projects in Mercurial repositories.

[Added 2011-03-10] Xcode 4 adds support for Git. Several of us have filed requests for Mercurial support; you should, too, if you want it.

There is a nice GUI frontend for Mercurial on Mac called MacHG: http://jasonfharris.com/machg/

It is free and very nice IMHO.

Xcode's Subversion support is pretty good. 90% of the SVN activities I perform are easily doable from Xcode. For the other few things I just fire up Terminal.

There are a couple things in their SVN client implementation that are annoying:

• The code that checks to see which local files have been modified seems to run on a background timer, and its pretty latent. Sometime it takes 5 minutes for Xcode to show a file as modified. The same thing is even more exaggerated w/r/t remote modifications.
• Sometimes when you rename or delete a file that isn't under source control, a dialog will appear, asking "Do you wish to [rename/delete] this file in SVN as well?" And the options are "Yes" or "Cancel." You choose Yes out of desperation only to be presented with a well-deserved SVN error.

Overall, I'd recommend it.