This is a great device. I have owned it for a few years now, using it daily, and it still works and looks like new.
I use it to control what sounds are sent to the monitor and to which monitors. It can A/B seemlessly between inputs and outputs, at the press of a button. It also has two simulatenous outputs (main and cue) that can each use a different input. Furthermore, it has two headphone outputs that can switch between main and cue at will. All volume knobs are stepped, except for the main one, which is continuous. They all feel very solid.
The meter can be calibrated to match your DAW or some other device. To do this, you send a sine wave signal at -18db from the device you want to match, then you hold the "calibrate" button and done! Easy! When the meter peaks, it will hold the red peak light until you press "Clear peak".
The additional features for mute, dim, and mono are also valuable. They only affect the speaker outputs.
It also has a microphone that is activated while the talkback button is pressed, which automatically activates the dim function on the speaker outputs. I have never used this feature so I can't comment on it.
The speaker selections works so that when you press A or B, the output switches to the speaker you selected, but C works independently. So these are the possible combinations you can have: A, B, C, A+C, B+C. It's worth noting while they were meant as outputs for speakers, you can use them for anything else.
All together it makes for a very convenient piece of gear. I wouldn't be without it!