Probably a little early to tell just now, but it's worth monitoring. Have you seen anything change?

Not yet. SER and SEL are confirming it based on a John Mueller tweet.
The tweet: [twitter.com...]

I said earlier this week it was likely one of those slow-roller updates like they used to be back in the day. Looks like that may be the case.

The date to start was known long ago, but the impact will take time to recognise. Those still running the interstitials will be the ones to suffer, and i'd like to know how they are doing. However, I wonder if we'll ever hear from them.

This seems to be a very low-impact update. Despite the majority of the site already being re-crawled since the date, one of my large sites with a giant interstitial ad didn't budge one bit.

I know one of our smaller properties took a big dip over the past week, but traffic is still good.

I've heard unconfirmed reports of people losing 80%+ traffic, removed their interstitials and traffic popped back up the next day.

I agree, engine. We'll likely not hear from them.

Has anyone started to see decreased rankings on pages or decreased mobile search traffic yet?

As Lake noted, the impact will be very low. Only those in spam filled niches will probably see any changes.

I disagree on the spam part from what I've been hearing since I first posted this last week. But I don't have hard data and the two that I have seen personally I'm not at liberty to discuss. But those two are certainly not spam sites. They're typically top-of-Google niche news sites.

If sites with interstitial ads fall in the rankings, then some other sites could climb above them and gain traffic.

There are several prominent news sites I've been watching with noticeable interstitials. One appears to have dropped the overlay... the other hasn't. It will be interesting to see if they get hit. IMO, they've deserved to get hit for a while now.

There are several prominent news sites I've been watching with noticeable interstitials

I just saw one on an extremely well-known, very prominent, news site.

Do you think that some sites could be so important that Google wouldn't dare to demote them? In other words, what about giving manual exemptions to the largest most-important websites?

in all events, it's sure that Google doesn't apply the exact same rules/criteria to all sites.

Worth remembering that people "expect" to see some sites for certain types of query. Google needs to return them, or people will think Google is broken.

Pretty sure that's why the Interflora penalty didn't stick- it made Google look bad. Ditto BMW.

[sitepronews.com...]

[edited by: goodroi at 12:19 am (utc) on Jan 26, 2017]
[edit reason] Splicing together same topic, different threads [/edit]

how to find our website is penalized or not ?

Your search traffic from mobile devices would likely have gone down fairly dramatically. Look at yur analytics and segment your traffic by mobile visitors from Google, looking 30 days before 10-Jan and after.