The CLP-620ND has two issues on xubuntu 12.10:
1) Can't print landscape from LibreOffice, instead it prints a truncated portrait page.
2) Prints one page fine then the next will be a pile of paper with a short line of garbage (letters and symbols) at the top of each page.
Neither of these problems is anything to do with the driver! The latest driver works fine with these two workarounds:
The first is caused by the fact that LibreOffice has a new default printer language of PDF. Change this (in Printer / Properties / Device / Printer language type) to PostScript (Level from driver). This problem caused by the pdftopdf filter not autorotating the PDF which LibreOffice sends to CUPS, the fix avoids that filter.
The second issue is because CUPS isn't finishing the USB transaction properly. You can either switch off the printer after each job (!) or you can use this pair of commands:
lpadmin -p Samsung-Laser -R usb-unidir-default
lpadmin -p Samsung-Laser -o usb-no-reattach-default=true
This lasts across reboots. Also see https://bugs.launchpad.net/ubuntu/+source/cups/+bug/872483
for full details of this, and my comment 63 which is for the Samsung.