We have eventually got around to the 2013 upgrade of our production TFS server. It had been put off due to some tight delivery deadlines around Christmas.
The upgrade went fine, unlike out some previous ones we have had.
The upgrading of our team process templates, to add the new features, was greatly eased by using Feature4Tfs tool on CodePlex. This meant one command line call and all the projects were done (we had no significant process customisation) as opposed to visiting in team project in the admin console.
For now we are continuing to run with our TFS 2012 generation build and test controllers. These are working fine with 2013, so we can upgrade these when it is convenient, not all in a rush.