Brightness problem after upgrade to Ubuntu 13.10 on ASUS Slimbook X401U
You can change your brightness using xrandr like in this example.
sudo xrandr --output LVDS --brightness 0.3
Also you can try to switch to fglrx drivers and than switch back to the open source drivers. Switching between drivers in ubuntu unity is very easy. Just follow those steps in the UI: System Settings, Software and Updates, Additional Drivers.