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.
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.
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.
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.