I opted out during my last flight. After everything I had read, I decided I needed to experience it first hand. I was expecting a huge scene, with the TSA agent screaming, "OPT OUT! We've got an opt out here!", and then being manhandled, groped, and basically demeaned in front of other passengers.

After removing my shoes (the one part of airport security theatre I absolutely hate) and placing my belongings in those gray trays, I walked up to the TSA agent, who asked me multiple times if I had anything in my pockets-had I forgotten my wallet? Did I have my wrist watch on? Etc.

After the person in front of me had completed his scan, the TSA agent directed me to step forward. I asked if I could opt out. He responded with, "sure", and in his walkie-talkie, said, "I have a male-opt out." He then looked at me and said, "One second..."

He got a response back in his walkie talkie, and then directed me to walk through the scanner, pointing out that the scanner was not on.

On the other side, I was greeted by an older gentleman, who also pointed out the scanner was off. He asked if I would prefer being patted down in a private location. I declined. He then explained everything he would do: From the pat down, to using the back of his hand for the more "private" areas. I said, "okay...", honestly expecting the worst at this point.

He proceeded with the pat down at this point. He did my upper body first (arms, chest, back) and then went to my backside and said, "I'll now be using the back of my hand to pat your more private area" (my butt, basically...) I responded, "okay", and then with a brush, he ran his hand down the backside of my leg.

I'll be honest: My immediate thought was, "That's it? I've been grabbed worse in a club/bar..."

Of course, I still hadn't received the crotch check...

He then explained he would be patting down my leg. "Here goes...", I thought.

With the back of his hands, he patted down my upper thighs (no where near the crotch), and then wrapped his hands around my legs and went "up" until "contact" was made, but immediately moved down, patting the rest of my leg. He then moved on to the other.

Again, I've seen and experienced far worse contact made in a club/bar.

After that, he had to get his gloves scanned (similar to the band they use for laptops), and after everything came clear, he thanked me for cooperating. I thanked him in response, and went on my way.

While I realize it's entirely dependent on the TSA agent you're dealing with, as well as personal/emotional experience, my own experience wasn't nearly as bad as I had prepared myself for.

When I pointed this out to a friend of mine (via text), he responded, "Yeah, but you're not a hot chick..."

He probably has a point :)

A few observations:
Two of the four or five passengers who opted-in to get scanned, had to be patted down after - a similar experience I had (I went through the scanner once before, a mistake on my part, and had to be patted down after...)

The TSA agent made a comment during the pat down that surprised me: He wasn't a fan of the scanners himself. He said (paraphrasing here), "I've been reading about the radiation from these machines. You think passengers have it bad? I have to stand in front of this thing all day!"

Edit: Shoot, this is a lot longer than I had anticipated.
A tl;dr: I opted-out of the scanner out of my own curiosity. The airport I flew out of employed TSA agents who made the experience not nearly as bad as I had anticipated... Of course, YMMV.