Last week flew EWR to PBIA to SEA to EWR.
EWR was the terminal with the security stations arranged one after the other rather than side by side, and no NoS that I could see. No problem.
PBIA was all MMW and I opted out. Was relatively efficient -- pat down of me, but no luggage search. As is becoming typical, they made no effort to ensure that I could see my stuff, but by standing on tip toes I was able to see it through the MMW machine. Also they made me walk through the MMW machine to get to the pat down area. Pat down less thorough than those I have had in other countries.
SEA I was prepared for a mess based on posts here. They were in fact sending most people through the backscatter machines, but I noticed that the gatekeeper was asking people if they were traveling together or alone, and letting passengers traveling alone go through the WTMD if the NoS was backed up. Also that there was no way for me to indicate that I was opting out, they just put me in the line to the NoS. As I got to the front of the line the gatekeeper asked me if I was traveling alone, and then sent me through the WTMD (although I would have opted out at that point if directed through the NoS). Also, a TSA agent shoved my belongings into the luggage XRay long before I was at the front of the line, so I had no eyes on my luggage while I was in line.
... Because, as we all know, terrorists only travel in family groups. And never through EWR.