First, lots of successes...
I powered up my XR16, connected my iPhone to its built-in WiFi access point, and then opened the XAir Monitor Mixer app.
The app found and connected to the XR16 almost instantaneously.
It brought over the scribble strip values and displays them properly.
The previous state of the sliders also came over.
I didn't bother plugging anything into the mixer, so to see if things were working, I also connected my iPad and brought up the Behringer X Air app to see that if I moved a slider on the Monitor Mixer, it would also move on the iPad and vice-versa. It did, so I take that to mean things are working the way they should.
So, all-in-all, it pretty much works.
However, there were a couple issues...
When I click the gear, the monitor buses have no labels. I just see 4 solid yellow boxes (see attached screen grab).
Sometimes when I try to move a slider (makes no difference whether it's an individual channel or the master), it won't move. Then, after a few seconds, it will suddenly snap to the new position, or sometimes it won't move at all. If I wait long enough it will eventually "unstick" and start moving, though.
Although I had nothing plugged into the inputs, the VU on every channel shows green for about 1/8th of its height. Sometime it vibrates up and down 1 tick or so. Looks like what you'd see if the gain into the meter is way too high and you're just seeing the noise floor.
When viewing the mixer screen and I rotate the phone from portrait to landscape, the screen does a rotation animation, but ultimately just shows the same screen no matter the physical orientation of the phone. I guess it doesn't really matter, but it would nice if you could turn off the animation when the orientation of the phone changes since the screen doesn't actually change in appearance.