The reason people dont notice the light sensor working is because it is doing its job well! If you are viewing the screen in low light conditions it looks as good as it always did, but the actual backlight power is much lower.

You can see it working for yourself. Just you look at the screen in a brightly lit room, then put your thumb over the light sensor. After about 5 seconds you'll see it go much dimmer because it thinks you are in total darkness.