On my recent flight from Denver, I had my first experience with a new backscatter security scanner. And I have to say, I found it very disturbing.
First, it caught me by surprise, I was returning from a workshop, having a good conversation with a colleague, going through security on autopilot. I put my stuff on the x-ray belt and then the first unusual thing happened: the TSA guard asked if I anything in my pockets. "No metal" I said. "No anything" they clarified. Odd, I thought, and double checked. "OK, nothing" I affirmed, just my boarding pass in my hand. Then I was ushered towards a device next to the normal metal detector which looks like a clear vertical tube about the size of a broom closet (the photo in this LATimes article seems close).
Now unlike walking through a metal detector, where you basically walk through like you would a doorway, the backscatter process is much more controlled and time consuming. First they took my boarding pass. Then I had to step in and assume a fixed position with my hands in a triangle over my head (thumbs and index fingers touching - the photos I find online don't really show how high your hands have to be). I had to hold that position for several seconds while the device pivoted around me, closing the entrance I walked in and opening an exit towards the other side. While I was standing there all the images I'd seen online were going through my head and the thought that somewhere someone was now basically looking at me naked while I had to hold this pose. (It probably takes a moment for the image to develop so in fact they weren't looking at me yet.)
Then I was told to wait in the device for a few more seconds, presumably until someone verified they got a good image. The guard at the exit had a walkie-talkie they were using through the process to communicate with someone looking at the image - I don't recall if they got a clear signal to let me exit or if they just waited a fixed period of time. After I exited I figured I was done, but I was then directed to another spot, outside the scanner, with foot outlines on the floor, and had to wait for another 5-10 seconds until the guard got a "male subject clear" message from their walkie-talkie (actually, the first time the message was garbled and the guard had to ask for a repeat, hence my knowledge it's a two-way communication channel).
Then I was given back my boarding pass and was free to go collect my stuff from the x-ray belt, where it had been sitting for a while at this point. (I wonder what would keep someone from walking off with it? I certainly was in no position to keep an eye on it. It wasn't clear TSA was.)
So why did I find it disturbing? First, I have to say the TSA guards were courtesy through out, they did nothing to contribute to my discomfort besides being part of the process. I can think of three reasons why it bothered me: (1) from start to finish, it's much more controlled. Unlike the metal detector, where you walk through on your own volition, it is a series of steps where you stand in specific places and poses - you feel very tightly controlled. (2) It's very asymmetric: somewhere, someone you can't see is scrutinizing you and you don't even know where or who they are. (3) It's very revealing. Having seen pictures online, I know just how laid bare I was. I've never been strip searched, so I won't say it's comparable, but with the combination of being tightly controlled and knowing I'm being seen basically naked, it's the only analogy that comes to mind.
Why was I picked to go through the device? Best I could tell, they just picked the next person in line as soon as the scanner opened up.
Did it make me feel safer? No. From everything I read (google for "odds of dying from terrorism"), I know I'm much less likely to die from a car crash, cancer and many other things. My odds of being killed by terrorism on that flight from Denver I'm sure were much lower than my odds from dying from human error on the flight.
Would I do it again? You can decline to undergo the process, but based on my experience, don't expect this option to be offered, you'll probably have to assert it. There is some debate as to how you'll be treated if you do so. I did find it disturbing enough that if I'm not in a hurry, I will decline it and take the alternative.