There really isn't much you can say about this device, because it's boring, has no user controls bar an on-off switch, but is an essential part of our system.
In our band, the drummer and I have been on in-ears for a long time. I find I need a very specific mix (too many nasty harmonies) so I bought the P16M for our Behringer system. I'm happy - but the guitarist and keyboard player still make too much noise, and soundchecks were taking too long. In the end, they bought some Behringer small active speakers and attached them to their mic stands, but still soundchecks took too long, with everyone wanting too much, in too short a time. I bought another couple of P16M personal mixers, but discovered that they need powering and that was a bit of a pain.
The P16D Ultranet unit simply lets you connect each P16 mixer with the single cat5 cable. Power is supplied down the cable, making the wiring much, much simpler. Connection wise, the device sits in the rack next to the S16s, and connects to this unit with another Cat5 cable. You turn it on, plug in and there are no controls whatsoever.
The only downside I can think of is that Behringer don't make a rack mount kit - so mine is simple in the rack, gaffer taped to the top of the S16 - a bit of a bodge but fine in practice.
This unit will also connect to other behringer Ultranet devices if you don't have an X32 to drive it.
It's simply a distributer of the Ultranet audio and power.
You cannot use a normal network switch as the Ultranet system supports the DC power system to power the P16M's, and a network switch does not.
Can I recommend it? Of course - it makes wiring so much more simpler than trying to supply mains power near each mixer on stage.