I bought this because I sometimes have friends over for recording sessions. I wasn't previously able to monitor singers while they were singing, as I only have one headphone output. I wanted an easy solution that would allow me to monitor the recordings in realtime. I didn't want to spend much, because 95% of the time, I only need one headphone output for myself.
This box not only solves that problem, but also allows the singer(s) to set their headphone volume independently. I prefer to monitor at low volumes; many singers like to turn the volume up high. There's no need to compromise, as everyone gets their own volume knob and can set it to whatever is comfortable to them.
The knobs seem pretty decent, especially given the price. Mine are firm and feel good when you turn them. Other than the 4 knobs, there's not really much to the device. It does what it says on the tin, and that's it.
If you are looking for a cheap way to allow up to 4 musicians to monitor the *same* signal through headphones, you probably can't go wrong.