Usually reviews happen when people get their hands on products 😉 My guess would be the first will be start popping up 2-3 days before official release.
Apple decides the timeframe of the review embargo. That's why all the full reviews come out at the same time, usually a few days before launch, I think.
Some reviewers had a hands-on yesterday after the KeyNote. So some of the reviews will be brief based on their thoughts from yesterday. But user reviews will surface during the launch.