I upgraded from 2010.2 to Mageia 1, had one unfortunate issue (which I already fixed myself, anyone who remembers me will know that) ... I was using the nvidia driver before, after the upgrade the driver didn't work. And unlike Mandriva 2010.x, it didn't automatically direct me to the X11 configuration utility when X failed to start. So once I switched to a text mode terminal, I configured it to use the nouveau driver; once I got X running I reinstalled the nvidia driver and now everything's okay. Well, mostly - I don't see "Configure your Desktop" (KDE control center) in the menu.
Anyway, it would be best if you advise people who are upgrading to disable proprietary drivers in the old system prior to install, so they don't have to do that themselves.