I finally tested the SharePoint Server 2010 December 2010 CU package over the last couple of nights. The good news is that it actually worked (I’ve had trouble with August and October) and it has a load of fixes, particularly for the User Profile Service Application. The bad news is that it’s known to require restarting the User Profile Synchronisation Service after it completes. In my tests, I also had to temporarily re-add the Farm account as Local Admin and reboot before re-starting the service, after running the installer and the Products Configuration Wizard. It failed when I just tried to temporarily add the Farm account as local admin and log off/on again, so the reboot before re-starting the service is likely to be necessary.
UPDATE 19/2/2011: I got a comment from Spencer Harbar today (below) noting that restarting the SPTimer service is sufficient after temporarily adding the farm account as local admin. The reboot isn’t necessary to acquire the new rights although in my test I did need to reboot after running the installer.