I have a Clevo W350ST with Ubuntu Saucy, had installed the Bumblebee PPA and experienced the same problem with the brightness keys not working.
However, by removing the bumblebee PPA, purging bumblebee, re-installing it from the Saucy repos, and then adding kernel boot parameters "acpi_backlight=vendor" and "acpi=force", and now the Fn-F8/F8 correctly alter the brightness correctly.
I have a Clevo W350ST with Ubuntu Saucy, had installed the Bumblebee PPA and experienced the same problem with the brightness keys not working.
However, by removing the bumblebee PPA, purging bumblebee, re-installing it from the Saucy repos, and then adding kernel boot parameters "acpi_backlight =vendor" and "acpi=force", and now the Fn-F8/F8 correctly alter the brightness correctly.