Yesterday I ran the graphical program that offered to upgrade my Ubuntu 12.10 system to 13.04 and it appeared to run flawlessly. By that I mean it presented no errors or warnings, ran to completion, and booted into 13.04.
When I enter my password into the GUI to log in, I see a flash of a somewhat jumbled TEXT screen, and then the system returns to the login screen. If I enter the WRONG password, it just tells me that my password is wrong. Also, I verified that I CAN still log in across the network using SSH (though the MOTD tells me that I'm running 13.04 and that an upgrade to 13.04 is available!)
This isn't a huge problem because I use the machine as a network server and almost NEVER use the GUI (also, I utterly DESPISE 'unity') but since that's the recommended way to upgrade, I'll need it again someday.
Oh, I also tried running
sudo apt-get update and then
sudo apt-get upgrade just to be sure that everything was correctly updated and it found nothing to upgrade.