Under Ubuntu 14.04, with a Lenovo W510 using NVIDIA proprietary drivers, brightness controls are not working. Pressing [Fn][Home] or [Fn][End] does change the value of /sys/class/backlight/acpi_video0/brightness, but nothing on screen changes. It does work with the Nouveau open source driver.
Workaround: Jump to a console with [CTRL][ALT][F1], change the brightness with the keys [Fn][Home] and [Fn][End] and return to the X11 session with [CTRL][ALT][F7].
A normal consumer would not know how to correct this. This used to work under Ubuntu 12.04 LTS.
I found the following web pages showing others had simillar problems, even with Intel based graphics, but none of the solution helped me. xbacklight for instance does not work.
More details are available in the bug report.