They are under a non-disclosure agreement until a time specified by Apple. For the Air 2 the embargo was lifted at 9PM EST the night before release, so I'm assuming that's how it will work here too.Certainly some reviewers must have it by now. Surprised no leaks on Twitter etc. to the effect of one having it and that their review will be coming shortly.
When it goes on sale next week, the reviews will come one after one. I am waiting for them as well for the Apple Pencil.