I tried AR on both an iPad Pro (12") and an iPhone 5S, both running the latest iOS (11.4.1). Both worked without problems.
On a moonless night, the camera shows only the very brightest stars and planets, and then faintly -- as you might expect, given the camera's sensitivity. This made it a little tricky to calibrate the AR view left-right because I had to jiggle the device to identify the image of, say, Saturn, but once done it held calibration for the duration of the tests. Blending the two views worked fine, although with the sky view bright enough to read labels and such it is even harder to make out the actual night sky objects.
Objects line up vertically only in one position as the device is tilted -- presumably due to differences in the map projection and the camera's distortions.
Overall: pretty cool, but I'm doubtful I would use this feature much, mainly because I already know my way around the sky. I worry that someone who didn't would be confused by the relative displacement of the two views, and would not be able to find an object bright enough to get the fields to align left-right.