It's really not bright. I'd guess it's probably less bright than the keyboard backlight, which also isn't bright (but is at least dimmable). It's so not bright that I've literally never noticed the thing at night in well over a year of regular use. Whatever number of lumens it outputs is so far below the brightness of the main display that it's all but unnoticeable in use.
I cover my monitor's power LED as well as the status screen on the receiver for my wireless headset. I run F.lux on the reddest settings when I'm within 1-2 hours of bedtime. (and after that I don't run any screens)
I have no issues with daylight brightness, but must carefully control lighting at night due to a broken circadian rhythm. Trust me, I don't need to own it to know that it's too bright. Especially if F.lux is not supported, which it is not last time I checked.